after upgrading to ventura 13.6.6 login to Teams is impossible and Safari complains about non standard compliant certificates

After upgrading to Ventura 13.6.6 (on an Macbook Pro 14.2 (2017) it's no longer possible to login to Microsoft Teams.

I'm able to choose the account to use, but after a short period of time I always get back to the page where I can select the account to use.

If I try to create a new account - the result is "No Network Connection Please check your network settings and try again. [2604]"

Due to Microsoft Support forums this is because of TLS 1.2 being disabled.

But there's no possibility on a Mac to enable this explicitly, because it's enabled by default since Safari 7 ...

The second strange phenomenon I see is Safari complaining about many non "standard-compliant" certificates and thus rejecting to establish a connection to the website. The certificates themselves are absolutely okay as they were accepted by Safari before the update - and are still accepted by all (i.e. Chrome, Chrome Canary, Firefox, Opera) other browsers after the update ...

Any help on this would be highly appreciated.

Thanks in advance,

uli