I recently started to experience issue during tests of my flask application. I even tried to restart some old job to confirm that the issue is not caused by the PR that is being tested. The tox tests are working fine on my Fedora 34 machine, but failing on Travis - CI.
I’m wondering If you dump the traceback on a SIGSEGV signal if you’d get the same or similar output? The other thing I noticed is you’re still using .org, I recommend migrating at this point to .com.
Most of the Pallets’ projects have been upgraded with breaking changes recently. I’d suggest you to pin Flask < 2 maybe to see if my guess is right — I think your local .tox/ folder has a pre-cached env that has old dep versions while in Travis some have been updated because they are not pinned in the requirements/contraints files.