Hi @Pratyush Sharma,
Thanks for your hint that Enpass is just using the system wide root certificate store. It seems like all browsers on all my "not working" devices have own CAs built in so it just works.
I then tried to figure it out using a connect of openssl on my ubuntu 20.04 machine and discovered that my server was sending the wrong (old) intermediate certificate. So the browsers seem to "know" the intermediates already and trust them while the root devices obviously do not.
After changing my QNAP NAS to use the correct new intermediate, it works like a charme again.
Thank you!
BRs,
hummels151