I wanted to upgrade the forum using the package manager. This revealed errors in five files. These were files that had been manually manipulated (including QueryString.php and Subs.php; the other files had been modified for the Mod Autorotation).
I replaced these files with the original files from forum version 2.1.4. This resulted in an error-free starting point (see also image: SMF starting situation), and I performed the update.
...the result -> Image: SMF result.jpg
What I have not mentioned so far: I have also installed two additional themes, which (due to their age) also contain errors for the new forum software 2.1.5. So far, I have completely ignored these errors in order to look at these themes later. If a theme became unusable as a result, I deleted it.
Could this be related to the fact that I have a total of 3 themes (including the default theme) and a supposed triple installation?
During my last attempt, without replacing the 5 files with unmodified files, it seemed to be that the module"SMF_2.1.5 Update" was only installed twice. So it can't be due to the themes after all.
Because the Dev from Autorotation recommended that I should only be one version of SMF_2-1-5 update, I should tried to clean it up according to his recommendations (Link (https://www.simplemachines.org/community/index.php?msg=4191186)).
An issue was logged based on the earlier reports: https://github.com/SimpleMachines/SMF/issues/8689
I would keep one & delete the others.
I would next confirm versions are ok by running the version checker. (It is safe to ignore version warnings on languages.)
That was easy. Just delete the duplicates from the packages folder. Thank you.
I just used the delete button.
I get a warning saying "Module is installed." I didn't want to risk that, so I thought it would be safer to simply delete it from the package folder. Here you can also see which packages have been uploaded "too many times" (package name.zip, package name-1.zip, package name-2.zip).