TimeMachine backup fails even though there is fee space

Hi,

for a few days my TM backup started to fail every time. On Scale 25.04 the dataset currently shows the following:

Space Available to Dataset: 48.22 GiB

Applied Dataset Quota: 500 GiB (applies to descendants)

In SMB share the TM quota is 0.

macOS Tahoe shows 51.78 GB available.

Yet backup consistently fails with:

“%@” could not be backed up because there was not enough space on the backup disk even after older backups were deleted. Select a larger backup disk or make the backup smaller by excluding files.

Any suggestions as to how to debug this or fix?