Greetings!
I like to assume that the server must be down for dowbloading all the default pak files from sourceforge since i ran into the exact same problem installing the port driectly from google playstore this year. My solution was to go to an apk mirror like ApkPure and download an XAPK file containing all the necessary files from the latest release of the port. That site also has instructions on how to install XAPK files. Since you said you moved you attempted to copy the original files from another device that had Open Arena installed (i'm assuming its a pc) you could technically skip this step, but the android port has a unique Pak7 pk3 file that has all the touch controls and options regarding the touchscreen ui stuff.
Here's the tricky part sorta: the app is looking for a specific file to confirm that the assets have been downloaded in the form of a .obb file i think, and it should be located at the following filepath in the event that the apk had properly installed and downloaded all the assets:
storage/emulated/0/android/obb
The file in question has the file name " main.8828.ws.openarena.sdl.obb" located in
the folder named " ws.openarena.sdl". If memory serves me right, once you downloaded the XAPK, this file should be included along with it since an XAPK is essentially an APK with all its assets packaged together. We best wait for Pelya for support since he was the one who made the port and is m9re knowledgable, but you can try my suggestions while waiting if you have time to spare.
As for bots and new skins i have done a good amount of surveying myself, and the biggest collection so far would be either of these two places:
https://ws.q3df.org/models/ (worldspawn archive, where there are also a lot of skins for q3 characters, and for obvious reasons would not work with the Open arena characters)
http://ioquake3.org/extras/models/ (supposedly an old archived Polycount page that also has some weapon skins and sounds. Maybe even sprites/effects but i dont recall all too well)
You can download maps from Lvlworld
https://lvlworld.com/And if you want to legally frag characters from Quake, specifically Q1 and Q2, someone remade the Enforcer and Guard enemies respectfully from scratch:
http://www.tabun.nl/downloads.phpFilefront, fileplanet and Planet quake have had an abundance of models, especially Planet Quake.
The sad thing is filefront has most of the same items that worldspawn has if not less, and Planet Quake which had literally the complete motherload of unique and great quality custom character models and more was shutdown when gamespy was shutdown,all links leading to IGN. I'm assuming there's another archive site somewhere that had some of the other unique models intact.
I hope this could be of any use to you, and i bid thee well in getting OA android port to work!