Thunderbird won't start since installing Avast One gold - Help please

Thunderbird won’t start (AVAST one? – help please (desperate)

Dear all

I have been using thunderbird for many years – no problems.

Since yesterday, It does not start any more.

  • When I start it, I get a white blank window – when I click on the windows top edge, it says “Mozilla thunderbird startpage (not reacting)” – and after a few seconds, it closes.
  • Yesterday, just before I noticed the problem, I installed AVAST Gold (Virus scanner/ system optimizer) – and let it scan and optimize my system. – Would think that this has caused the problem, since before I had no problem)
    • Before I installed this premium GOLD version, I had AVAST free version running – no problem, so the problem might likely come from one of the premium functions in the gold version.
  • I undid (almost) all the cleaning actions AVAST had performed – Problem is still there
  • I rebooted with AVAST turned off - problem is still there
  • I uninstalled / reinstalled thunderbird - Problem is still there
  • Thunderbrid profile files (mails etc.) are still in place

General Info

I am running Windows 11 – there was no windows update in the last 2 weeks

Any recommendation what I could try – Thanks for your help