I was eventually able to fix it myself. See this thread : eas submit — The Android App Bundle was signed with the wrong key - #9 by flexbox
The trick is to have the jks file have a storetype of ‘jks’ and not ‘PKCS12’. Also make sure that both passwords (keystore and key) are the same, otherwise it won’t work.
It would be nice if the expo website somehow gave a better error message than " Unexpected server error" though. That would’ve saved me some time 
cheers