Sadly this still happens in Unity 2019.4.15f1, did just search in the project browser (not by type). It also happen during playtime (randomly) which causes the "Error Pause" to trigger. That's very annoying.

Request error (error):
0x00007FF70BCF0A7C (Unity) StackWalker::GetCurrentCallstack
0x00007FF70BCF4A31 (Unity) StackWalker::ShowCallstack
0x00007FF70A3B4EA5 (Unity) GetStacktrace
0x00007FF70C978BCE (Unity) DebugStringToFile
0x00007FF70BD4FE45 (Unity) DebugLogHandler_CUSTOM_Internal_Log
0x000002149090987B (Mono JIT Code) (wrapper managed-to-native) UnityEngine.DebugLogHandler:Internal_Log (UnityEngine.LogType,UnityEngine.LogOption,string,UnityEngine.Object)
0x00000214909096FB (Mono JIT Code) UnityEngine.DebugLogHandler:LogFormat (UnityEngine.LogType,UnityEngine.Object,string,object[])
0x0000021490908B7E (Mono JIT Code) UnityEngine.Logger:Log (UnityEngine.LogType,object)
0x00000214909087CA (Mono JIT Code) UnityEngine.Debug:LogError (object)
0x00000214908FE4BB (Mono JIT Code) UnityEditor.AssetStoreClient:ParseContent (UnityEditor.IAsyncHTTPClient)
0x00000214908FD993 (Mono JIT Code) UnityEditor.AssetStoreClient/<>c__DisplayClass45_0:<WrapJsonCallback>b__0 (UnityEditor.IAsyncHTTPClient)
0x00000214908FD5A3 (Mono JIT Code) UnityEditor.AsyncHTTPClient:Done (UnityEditor.AsyncHTTPClient/State,int)
0x0000021425F26C8F (Mono JIT Code) (wrapper runtime-invoke) <Module>:runtime_invoke_void__this___int_int (object,intptr,intptr,intptr)
0x00007FFA59D6D6B0 (mono-2.0-bdwgc) [mini-runtime.c:2809] mono_jit_runtime_invoke
0x00007FFA59CF2912 (mono-2.0-bdwgc) [object.c:2921] do_runtime_invoke
0x00007FFA59CFB96F (mono-2.0-bdwgc) [object.c:2968] mono_runtime_invoke
0x00007FF70BC630EE (Unity) scripting_method_invoke
0x00007FF70BC5CE1D (Unity) ScriptingInvocation::Invoke
0x00007FF70AA15708 (Unity) AsyncHTTPClientRequest::Done
0x00007FF70C91DB66 (Unity) CurlRequest::Check
0x00007FF70C91E129 (Unity) CurlRequestCheck
0x00007FF709A60E0B (Unity) Application::TickTimer
0x00007FF70A3DA905 (Unity) MainMessageLoop
0x00007FF70A3E4AE8 (Unity) WinMain
0x00007FF70D440192 (Unity) __scrt_common_main_seh
0x00007FFAB0867C24 (KERNEL32) BaseThreadInitThunk
0x00007FFAB280D4D1 (ntdll) RtlUserThreadStart

Happened to me in Unity 2020.1 while searching the project window as well. Had me stumped for a while.

good one <a href="https://www.lyricsauto.com">lyricsauto</a>

Using Unity 2019.3.10f and still not fixed

Digging further: this is caused by internal errors within Unity's backend service at shawarma.unity3d.com.

Here is a successful HTTP API call (captured using Fiddler) with request & response:

GET http://shawarma.unity3d.com/public-api/search/assets.json?unityversion=2017.3.1f1&skip_terms=1&q=bootcon&c=&l=&O=&N=&G= HTTP/1.1
Host: shawarma.unity3d.com
User-Agent: UnityEditor/2017.3.1f1 (Windows; U; Windows NT 10.0; en)
Accept: */*
Accept-Encoding: identity
Connection: Keep-Alive
X-Unity-Session: 26c4202eb475d02864b40827dfff11a14657aa41548501f33da35ce3a96c421d36f00c92b1d61c5c4c79942712fe083b1f0d8125c825bc7cac67115a

HTTP/1.1 200 OK
Server: Apache/2.2.22 (Debian)
Cache-Control: max-age=0, no-store
X-Kharma-CacheInfo: uncacheable
Vary: Accept-Encoding
Content-Type: text/plain
Content-Length: 502
Accept-Ranges: bytes
Date: Wed, 28 Mar 2018 07:03:39 GMT
X-Varnish: 3565388464
Age: 0
Via: 1.1 varnish
Set-Cookie: SERVERID=varnish02; path=/
Cache-control: private

{"query":{"limits":[],"offsets":[],"query":"bootcon","groups":[],"class":[],"labels":[]},"status":"ok","groups":[{"matches":[],"time":"0.000","name":"free","total":0,"words":{"bootcon":{"hits":0,"docs":0},"free":{"hits":200186,"docs":173406}},"total_found":0,"error":"","warning":"","label":"Free Assets"},{"matches":[],"time":"0.000","name":"paid","total":0,"words":{"paid":{"hits":4262110,"docs":4262028},"bootcon":{"hits":0,"docs":0}},"total_found":0,"error":"","warning":"","label":"Paid Assets"}]}

Here is a failed API call -- notice that the request is identical to the successful call:

GET http://shawarma.unity3d.com/public-api/search/assets.json?unityversion=2017.3.1f1&skip_terms=1&q=bootcon&c=&l=&O=&N=&G= HTTP/1.1
Host: shawarma.unity3d.com
User-Agent: UnityEditor/2017.3.1f1 (Windows; U; Windows NT 10.0; en)
Accept: */*
Accept-Encoding: identity
Connection: Keep-Alive
X-Unity-Session: 26c4202eb475d02864b40827dfff11a14657aa41548501f33da35ce3a96c421d36f00c92b1d61c5c4c79942712fe083b1f0d8125c825bc7cac67115a

HTTP/1.1 200 OK
Server: Apache/2.2.22 (Debian)
Cache-Control: max-age=0, no-store
X-Kharma-CacheInfo: uncacheable
Vary: Accept-Encoding
Content-Type: text/plain
Content-Length: 208
Accept-Ranges: bytes
Date: Wed, 28 Mar 2018 06:56:26 GMT
X-Varnish: 814532282
Age: 0
Via: 1.1 varnish
Set-Cookie: SERVERID=varnish01; path=/
Cache-control: private

{"query":{"limits":[],"offsets":[],"query":"bootcon","groups":[],"class":[],"labels":[]},"status":"error","error":"Failed to open connection to localhost:9312: Connection refused... ConnectRetries exceed..."}

And that error message? "Failed to open connection to localhost:9312"? That is an error when one service in the backend fails talking to another in the same backend.

Port 9312 is used by the Sphinx search server (http://sphinxsearch.com/).

It seems that the frontend APIs are hitting the Sphinx search server more often than it can respond, and the random search server errors are leaking through to the Unity Editor clients.

Can you please not leak those errors through to the Unity Editor clients?

This is happening randomly to me as well. It happens even when I do not have any Asset Store window open. Sometimes it happens shortly after I hit "Play" in the inspector. It makes me and others on my team think that the game is broken, when it isn't. It is extra annoying because it is an error outside of our control.

The error message originates from the Asset Store backend. Starting with this line: https://github.com/Unity-Technologies/UnityCsReference/blob/master/Editor/Mono/AssetStore/AssetStoreClient.cs#L711
... the call to ParseJson() will set the 'status' string to "error". In other words, the Asset Store backend has delivered a well-formed JSON which either contains status = "error", or error = "error".

A better fix is:

In Windows go to the files of your project as for example the folder ProjectSettings and check inside the security properties files as for example ProjectVersion.txt or any other file out from assets folder. And look if your user has read and WRITE permission. If not , inside the properties in the tab security, "edit" the settings to write or total control.

A fix is:

Edit -> Preferences -> DISABLE Show Asset Store Search Hits
Then close Unity and any text editors you're using with unity. And open them up again.

The error should be gone.

But I do not know the consequences of disabling Asset Store Search Hits

Same problem in Unity 5.4.4f1

I get that same error in many other cases, too, not just searching by type. Since I have "Error Pause" selected in my console, it's very disruptive.

I disabled showing asset store hits when searching to work around it.