Tried also with Microsoft Edge and it does the same…
Maybe is caused by the Thunkable aia?
Here it is the aia: thunkable_School_Life_Diary.aia (1,0 MB)
I also tried with a new project and it works… so is definitely the aia!
The new Firefox seems to have lots of issues with all App Inventor distributions Unfortunately, with Microsoft Edge I’m not even able to connect to the companion. I also have issues with specific Thunkable projects btw. I think they somehow got corrupted…
I think I found the problem: only renamed screens cause the issue.
So, to fix this you have to manually edit your aia.
Go into the file .scm of the renamed screen and change the value $name with the new name.
Do the same with all the occurences in blocks in the .bky file. (I suggest to use the Notepad++ feature Find and Replace)