I have the same error:
"notes = Browser: Mozilla/5.0 (X11; Linux x86_64; rv:121.0) Gecko/20100101 Firefox/121.0
foundIn = nb195d
faultData = com.google.gwt.core.client.JavaScriptException: (TypeError) : b.a.k is undefined
projectId = 4979497895854080"
I had been using appinventor 2 without any problems. I've been getting this error for a day now.
I reproduce the error with only the following:
* I create a new project "test_error".
* Without adding anything to the initial screen, I add a new screen with its default name.
* The error appears immediately.
I did the same thing again, but adding a label before creating a new screen (in case the empty screen causes problems):
* I create a new project "test_error_con_label".
* Add a label to Screen1
* Added a new screen with its default name.
* The error appears immediately.
I did not try with another browser (I see that they match Julian_Ha)