Severe Error: The backup index cannot be stored. The validation error code is 3. (4041500)

When I run a backup, it shows that it is writing to the media and then it just stops and gives me this error:

Severe Error: The backup index cannot be stored. The validation error code is 3. - check the ! for information Log Context: Validation error codes 0 - The index failed validation 2 - The index was not found or is of length 0 It could be offline and needs to be scanned OR the backup has exceeded the maximum index size Split the job 3 - The index is corrupt and cannot be read 4 - The index is corrupt and cannot be read (object pointers) 5 - The index is corrupt and cannot be read (-1 offsets) 6 - The maximum index size has been exceeded 7 - The index is corrupt (the end of record offset exceeds the index file size) 8 - The index size is unsupported (unable to seek to end of record offset)

I already tried reinstalling and re-adding the whole client. All my back up jobs for my other clients are automatically running on schedule without problems.