Couldn't load SIP module (QGIS doesn't load Python)

In a command window, type set > set.txt to get a list of all environment variables you have set. Your python installation may have set some values that QGIS does not like.

The PATH variable is save, because qgis.bat sets its own path variable, but PYTHONPATH or something else may be harmful.

Once you found a link to C:\Python27, go to the system properties and change the environment variable (not sure how the commands are in your language version).

pip3 uninstall PyQt5 worked for me

In my case, it turns out to be a conflict of different versions of python. I uninstalled python 3.7 after I installed the latest python 3.8. Somehow QGIS was still using python 3.7. And the folder python 3.7 reminds in the directory C:\Users\user\AppData\Roaming\Python.

Problem solved by deleting the python 3.7 folder.

BTW, it is a case on Windows