Daeraxa recommended me in the Discord #main channel to raise my issue here in the forum.
I am in the process of switching from a NextCloud appliance to a managed NextCloud at a provider where I already have another Joplin profile in operation.
I can switch between both profiles from my Windows clients without any problems.
When I have copied the approx. 2.4GBytes (zipped) of my main profile via nextcloud sync to the planned target directory in the new environment of the hosted NextCloud, I can change the URL, ID and password on both the Windows client and the Linux client and the clients connect to the new environment and start synchronising.
After that, when I change the URL, identifier and password in my Joplin 2.12.3 main profile on my Android 12, I get an error message saying that the credentials are not correct. However, I have changed these access data identically via copy-paste, as I did with the Joplin on Windows and the one on Linux.
The crazy thing is that I had already successfully tested the connection once in a new empty profile in the Android app with this data - at that time I had not yet documented in detail which steps I had carried out when and in which order.
As I could not find an answer to this 25/08/2023 problem, I brought forward my investment in a new Android 13 smartphone. This Android 13 is now also running a Joplin 2.12.3 . So now I have two migrations to keep an eye on at the same time: Migration of my Joplin instance to a new NextCloud and migration or parallel operation of an Android 12 and an Android 13.
This way I can test accessing my Joplin directories in my new hosted NextCloud with the new Android 13 smartphone without compromising my productive profile in my Android 12 smartphone on my still productive on premise NextCloud on my small NextBox.
First, on the new Android 13 smartphone with newly installed Joplin, I successfully connected the new target environment of my future productive instance - i.e. exactly the target environment that could not be connected either as changed Hauals second profile from my Android 12 smartphone because of allegedly incorrect access data.
After that, I wanted to connect the second profile that runs successfully on my Android 12 with the new environment also as a second profile on my new Android 13. When I tried this, I again got the error message that my access data was allegedly wrong - which I had again entered identically via copy-paste.
Since then I have suspected that there is some kind of previously undiscovered error in the implementation of the profiles function in the Joplin Android App 2.12.3.