Cannot find old Time Machine backups

Recently, I had to backup my iMac from my latest Carbon Copy Cloner copy. (I have an external HD with two partitions: Time Machine and CCC, both some 750 GB; for various reasons, I choose the CCC backup.)

After that, Time Machine started to backup again. But after a while it stopped, claiming having too little space to complete: it needed 700 Gb but it found only 230 Gb.

The old TM backups had disappeared!

Yet, Disk Utility says the capacity of the TM drive is 750,82 GB, and that 232,89 GB is available.

So, the older backups must still be there… but I cannot approach them. Also, when right-clicking the TM icon in the Dock in order to find another TM disk, it does not find the original backups.

A hint, ayone?