Hi DanielSame problem here. The problem appeared when Python was upgraded to version 3.9.
Apparently the (deprecated) threading.Thread.isAlive() function is removed in Python 3.9 and one should use the threading.Thread.is_alive() function.
Just replacing all occurences of 'isAlive' in /usr/share/hplip/scan/sane.py by 'is_alive' solved the problem for me (up-to-date Debian Testing).
Best regards Maarten