Execution as unprivileged users to perform a complete backup in conflict with Linux permissions

This means that restic is not even allowed to open the keys/ folder read only. I don’t know what’s going on, I can only see that restic is trying and the system (or the kernel) denies it.

Once upon a time, it had been in that state. Now you wrote about system, kernel.

This is what you need to fix:

You have to be able to do that as your user. You have to be able to create directories and files inside of the restic repository.
No other commands or concatenations like you did previously involved - if this fails, then you need to figure out why and nothing else.
What fd0 means is that he is suggesting the same - that something on your system if blocking this access.