Long loading with adventure sync on
When adventure sync is enable the Ingress app needs a very long time to load.
It will show the startup screen for some minutes and then after displaying a "login required" message it asks for the login information. This can loop for several times until it successfully starts. It can take from five minutes up to one hour.
If adventure sync is disabled, the app will start just fine.
The following steps do not help with the issue: Killing the app, restarting the phone, cleaning the cache, changing login between Google and Facebook, changing networks (wifi and mobile data) and various combinations.
Clearing the app data fixes it, because then adventure sync is also disabled. But as soon adventure sync is enabled again, then the long loading times come back.
Ingress version 2.108.1 on Android 10.
Note 1: Someone in this thread apparently had the same problem, but it was probably not further investigated.
Note 2: I have exactly the same issue with the Pokemon GO app and adventure sync. This indicates some core problem with the adventure sync implementation.