Serialization error: bulk data

Hi All.
I’ve experience some problems with saving a project after swapping for 4.20 to 4.21.

After rendering lightmass I can’t save my project. I’ve get an error around 53% “Serialization Error: Action Needed - Corrupt data found, please verify your installation”

Any suggestion ?
I’ve reinstalled 4.21 from scratch.
Regards

Bart