The last backup failed

Backup and Restore Center, Back up files ends up immediately with message
"The last backup failed"
Details tells me "...The latest backup set can not be found. Make sure it is
accessible and it has not been deleted. Otherwise create a new full backup.
(0x8100000A)..."

As far as I can tell, the latest backup set can be found (e.g. by using
explorer), and it is accessible (again using explorer).

I have tried to make a full backup, that works fine, but does not solve the
problem with incremental backup.

I have tried to change permissions, taking/assigning ownership (from SYSTEM
originally) etc. but nothing helps.

The backup set has been running fine from April 27 until June 27 (exactly 2
months !!), is there some time limitation or number of file limitation or
similar ? Should I manually clean the older directories ?