It happens once in a while - I'm using WhatsApp (far from backup hours) and it just starts backing up and not letting me to use the app for several minutes.
The last backup was successful and finished hours before it happened, and after that backup popup message, it didn't backed up to Google Drive as usual - so only a local backup.
My guess was that it's kind of "flushing the buffer" to save the current state, maybe as a result of too much new data since the last backup.
Why and when does it happen?
Anything to do about it? (Except from killing the app)
EDIT: Added two more screenshots. It keeps happening and also with the Google Drive backup.
Device: LG G5, android 7.0, custom ROM, no root