Hi all, just sharing this in case someone else will bump into the same issue.
After reinstalling /e/OS some time ago, the Recent Apps button in the Android menu (bottom right corner) did no longer function.
I tried many things, waited for updates, changed settings in the menu, tried different launcers, deinstalled apps, but to no avail. Apart from that, my phone functioned perfectly.
At some point I found out that my /data map was not mounted.
This was fixed via TWRP (fix nr 2 on this help page).
After rebooting, my phone started encrypting and then asked me to set up my credentials and settings.
(Data and settings were erased, so I was glad to have a backup.)
The Recent Apps button now functions fine.