Steps to repro:
1. Download the attached project and open "SampleScene" scene;
2. Clear Baked Data in the Lighting window;
3. Add reflection probe gameobject in the Hierarchy;
4. In the Reflection Probe Inspector press the Bake button to bake GI of the whole scene and reflection probes;
5. During the bake, once the ETA is visible, in the Lighting window Press "New Lighting Settings" button;
6. Once the new lighting settings asset is created wait until the GI is baked with the new settings asset assigned;
7. Once the baking is done, duplicate the reflection probe and move it somewhere in the scene;
8. In the Inspector of the duplicated reflection probe press Bake button and wait until the individual probe is baked;
9. When the bake is done, press Generate Lighting button in the Lighting window

Expected result:
The GI bake is started successfully.

Actual result:
The baking is not started and is stalled by the following warning message:
"To bake lighting, close all other instances of the Unity Editor on this computer."
even though only one Editor instance is opened. See attached video and Editor.log file.
Regression introduced after the branch lighting/1311231 would be merged with trunk.

Workaround is to restart Editor.

Notes:
- Reproducible in Version 2021.2.0a4 (fdccaf6d5eb7), branch: lighting/1311231;
- Wasn't able to repro in Version 2021.2.0a2 (14d497ab496d) Branch: trunk and Version 2021.2.0a4 (100ff7068915) Branch: trunk (because of the blocking trunk case https://fogbugz.unity3d.com/f/cases/1311231 )
- The baking stalls either in the new scene or in the current scene after following the repro steps;
- The only workaround is to restart the Editor;
- Whilst it looks like a regression, it should still be in fact caused by the SkyManager refactor PR: https://ono.unity3d.com/unity/unity/pull-request/118728/_/lighting/skymanager_refactor;
- Reproducible on OSX.