Here's the promised "detailed sequence of events" and possible problem causes I have been thinking about/investigated:
Two days ago I successfully flew the drone (session #7, a session being two flights as i have 2 accus, no problems encountered in previous sessions except for the occasional "connection lost" message from which it always recovered autonomously) until, 50% into the second accu, I lost FPV. Quite obviously the Range Extender had run out of battery as both lights were dead. I brought the drone down safely (visual flight rules) and recharged the Range Extender immediately when I got home. I don't think that this is in any way related to the problem I see now but thought perhaps I should mention it for completeness.
The weather remained good yesterday so I went out again, the first time with a rather detailed plan of what I wanted to do (waypoints, camera angles, altitude, drone orientation). On takeoff I noted that while I had a FPV picture on my phone I was missing the altitude/distance information on the display. I thought I would just do the first sequence of planned photo shots (single waypoint, single altitude, a few drone orientations & camera angles) before bringing it down again. To my surprise I got a message "SD card missing" (or so) when I wanted to take the first picture. Silently, I cursed myself for having forgotten to re-insert the card after I had downloaded the pictures from the day before, but on inspection after landing it turned out that the card was there and safely locked in its slot. I turned everything off and restarted powering up from scratch. When I turned on the DJI app on the Phone it said "Phantom connection broken". I could see the phantom on the list of WIFI devices but could not connect it: "Internet not available". After 15 minutes of retries without success and with cold fingers I gave up.
Today I decided to do a dry-test on the kitchen table. For a short period I could connect the Phone to the drone and saw a picture of the living room, then the Phone autonomously reconnected to my WLAN and lost the connection to the drone, leaving the message "Internet not available". I deleted my WLAN from the list of WIFI sources but that didn't help either. I re-inserted my WLAN into the WIFI list and again I got a brief glimpse of the living room before the connection to the Phantom was broken and the Phone reconnected to the WLAN. I went out of the house to revisit the exact place from which I had successfully flown a few days ago, I could see the Phantom on the WIFI list but could not connect because internet was not available. A few minutes ago I repeated the kitchen table experiment to make sure I had the story straight: This time I connected to the phantom for a brief instant, got the message "Please remove gimbal clamp" (which indeed I had forgotten to do), then the message disappeared and WIFI had disconnected autonomously from the Phantom.
All this led me to believe that somehow (i) my WLAN at home "overpowered" the WIFI connection to the drone (so moving away from my router and going out into the fields would solve the problem) but that (ii) my provider blocked access to the internet when not close to my router because I had used up my quota.
Does this make sense to you ? I'm almost certain it has something to do with my Phone settings... Perhaps I should do the kitchen table test with the Phone set to airline mode as suggested by DaveMoi?
Thanks for reading this to the end...
Johannes