Several days now: When connected to WLAN or Data Network after 5 or 10 minutes of connection the top of FP1 on back side (where camera resides) gets hot. After a while I get a waring message about the high temperature of the battery - but the battery isn’t hot - its the the case on top.
You can remove dust, when you open up the phone. Be aware that FP1 is not as sturdy inside as FP2 and pieces can be damaged easily (I broke my camera for example).
Ok - found a process in the background (app), which I didn’t start recently but which used a lot of resources - mainly for connection. This explains the hot top of the case only when online (WLAN or data network). So I killed and de-installed this app - and FP1 stays “cool” now. But I’ll monitor the situation even longer.