I have myself fought this issue a couple of times and I have seen it on the forum too.
When the backup is in progress there seems to be no way of knowing and backups can take a long time, so its hard to tell if or when you are affected.
If you then try to start a backup when one is in progress, then you get weird errors, that do not really tell you anything.
I would like a backup in progress indicator.
It would be nice if it was a progress bar or a sensor, but at the very least just an indicator on the backup page in the Settings → System → Backup just to stop you from trying to start multiple backup simultaneously.
I have seen that many request progress indicators for restore, but this is for the backup process instead and an indicator would be enough.
I do, but I have been sometimes working a bit on my HA installation in the morning and not knowing that the Samba backup was still in progress from the nights run, so my backup attempts failed.
If the Samba fails, then it looks like the backup hangs a lot too and an indicator then would be nice too, so you knew that a backup was still in progress and something might be wrong then.
If it’s a backup using a custom software method, perhaps petition the author(s) to post a persistent notification that a back-up is occurring, and clear it when the back-up is complete, perhaps add another one if the back-up did not complete successfully.
This is something the add-on author should be able to do without much trouble…
(HA core as well, but as it is a lower priority thing, more likely to happen from the custom software side sooner.)