Time Machine error message "Disk is nearly full", but it's not
There is a discrepancy of what is informed as available space and what actually is available for recording. This is discovered by Time Machine when it tries to save data, so the short answer is corrupted data (in this case, data about how data is stored). Time Machine does a complex kind of backup that is not just a copy, so my suggestion is...


Make a copy (better a clone) of the backup and erase the Time Capsule disk - format the volume.
Restore the data. Make sure the copy got the "real" size, or the backup might be corrupted as well.
To monitor the process once started, it has to end within one hour, or the problem might come back.


In my experience, it was taking longer than one hour, so Time Machine started the second backup before ending the first one, leading to a mess! There is third-part solutions to change the Time Machine frequency (two hours or more).