@acitonic since re-installing your phone, have you ever used it to send or read an SMS?

SMS Backup+ needs to temporarily change the default SMS app to itself, so that it can make changes to the stored SMS messages, and then change the default SMS back to the other app.

So it initially asks the system which app is the current default SMS app; if you haven't chosen one then (it appears) that it's not coping well.

Try opening your normal SMS app, confirm it as the default SMS app, close it, then re-launch SMS Backup+.

Failing that, go to your system settings and confirm that you have something selected as your default SMS app.

@kurahaupo thank you for your reply!

Of course I did all this. Just tried it 2 more times but without success.
Coukd it be that it has to deal with the new OS? It's Android 15 or Lineage 22.1? Or maybe that my phone owns 2 sim cards? But only one of them is used for sms or put for standard.
Moreover my phone is rooted.

OK, perhaps the ability to check for and/or change the current default SMS app has become restricted in Android 15.

If this is the case, it will likely just require an addition to the required permissions list, rather than a code change, but I won't know until I've reviewed the Android release docs.

A new comment in #1104 notes that there seems to be a new “allow restricted settings” permission that has to be enabled before the “SMS” permission can be enabled. Could you try that please?