Storage is full for legacy G Suite account despite having 15+ Gb left

[NicoDC]

Hello,

I just started getting this message on all my google docs, despite still having ample space (see screenshots below). 

Does anyone have any idea? This is a legacy G suite account used for family purposes only.

I would be very grateful for any pointers... I'd be happy to upgrade to a Workspace, but can't pay $100s/month for that.

Nico