Thank you! I managed to install 3.1.6 on my android 6.0 tablet. Unfortunately my other tablet runs Android 5.0.2, so its currently stuck at Joplin 2.9.8.
Regardless, even after upgrading I still ran into connectivity issues. I was trying to connect to a Joplin Server 3.0.1. When I pressed "Check Synchronisation Configuration" I get "Network Request Error". I then opened up the browser to check if I could access the URL of the server and it turns out that on both devices the certificate store was out of date, preventing me from accessing the URL over SSL, which also affected Joplin.
After adding the server's certificate manually (yay, for old devices), Joplin could synchronize with Joplin Server on both devices once again!
Anyway, this quest made me wonder what to do in the long-term for my older devices - for both devices, if synchronization eventually stops working on them (e.g. due to joplin API changes), they risk ending up as electronic waste.. For the Android 6.x tablet its not too bad, since Joplin 3.x introduced manual importing in the android app - so I could potentially keep it manually updated (albeit tediously). If the import feature eventually is backported to the Joplin 2.x branch, it might just provide Android 5.x devices that extra final life. 
