So the issue indeed seems to be with Android 7 in some way, depending on how you configured your server, I'll just copy a github comment i accidentally made to the wrong place.
I can confirm that the f-droid version was affected, the offending nginx config is this:
other relevant ssl config lines:
So with that option commented out it's working fine, I'm on 188.8.131.52-ose from f-droid and android 7.
Apparently Android 7 supports only up to secp256r1, this was supposedly fixed in 7.1.1.
Aside that, a bit of a rant, why are gitlab issues not enabled and instead this forum is used as a bug report tool? I have to create yet another account and wade through 5 google captchas just to write this.