Certificate for this server is invalid; Certificate is not standards compliant

Hello,

Two weeks ago I updated my MacBook Air (MacBookAir8,2) from Ventura (13.7.3) to Sonoma (14.7.4), with no problems so far. Two days ago I updated my Mac Mini (Macmini8,1) from Monterey (12.7.6) to Sonoma (14.7.4), and now I have problems with certificates, particularly accessing some websites, downloading assets from them, and some email server problems. I checked the date/time on both machines.

This screenshot shows an example - any ideas as to what is causing the difference, and how to fix it? I can't (and from what I understand probably shouldn't) delete the root certificate.

Top is the Mini, bottom is the Air:

Many thanks for your help!