How to reproduce:
1. Set up a server and host a custom Unity Package 'CustomPackage.zip' (see notes for one of the ways to do it)
2. Open the attached project (case-1233905.zip)
3. Make sure that the custom Package got imported (there should be a 'com.random.test" package in the Package Manager window or under the Packages folder in the Project window)
4. Open the Standalone Profiler window (Window -> Analysis -> Profiler (Standalone Process))
5. Change the Target Selection to 'Editor' (the default is 'Playmode')

Expected result: the Profiler window starts recording Frames and analyzing data
Actual result: the Profiler window does not start recording Frames and a 'Sending message header failed' warning occurs in the Console window

Reproducible with: 2020.1.5f1, 2020.2.0b3, 2021.1.0a1
Could not test with: 2018.4.27f1, 2019.4.10f1 (Standalone Profiler not introduced)

Workaround:
The issue stops reproducing after exiting and reopening the Standalone Profiler a couple of times. The issue reappears after exiting and reopening the Editor

Notes:
Steps to host a local server:
1. Install Node.js (https://nodejs.org/en/), Python (https://www.python.org/downloads/) and nmp registry Verdaccio (https://github.com/verdaccio/verdaccio)
2. Navigate to the Verdaccio folder through the CLI and run it with a command 'verdaccio'
3. Open a second CLI instance and run the command 'npm adduser --registry http://localhost:4873'
4. Navigate to the custom Package folder through the CLI and run the command 'npm publish --registry http://localhost:4873'
5. In the browser check that the package is published at http://localhost:4873