To reproduce:
1. Create a new project.
2. Create a new script.
3. Make it into Collab project.
4. Do some changes inside script.
5. Do "Collab -> Revert asset", for that script.
6. Try to press Play button now.

Expected result: After reverting Script changes, Scripts are compiled and it's possible to enter PlayMode.
Actual result: PlayMode will not start, because Editor is waiting for scripts to be compiled.

Reproducible with: 5.5.0f3, 5.6.0b5

Workaround:
Right click in "Project window" or "Hierarchy", or Dock/Undock a window.