Please check if there are logs inside Android/data/com.nutomic.syncthingandroid/files on the internal storage. If you find any log files there, and especially panic logs, please upload them here.
Otherwise, you need to run an up-do-date Android version, which means basically installing a custom ROM. For example, DivestOS (https://divestos.org) supports Nexus 7 (2013).
The “generating keys” message is misleading. In the background, Syncthing is stuck in a loop with the relay listener shutting down and restarting as described above (and clearly seen in the logfile). You could try to disable relaying in the app settings, if you can even access them.