Regarding the Air Manager, I switched the Ultimaker off, unplugged the Air Manager and started Ultimaker again. We had the Air Manager mostly closed since we have this printer, the print worked well (for some things I set the extruder temperature somewhat lower).
Regarding the not-working camera-stream, I did not yet do systematic testing. Yesterday I wanted to confirm the state of a print, and couldn't, because the Ultimaker apparently wasn't listening on port 8080. Today when I switched the printer on again, the camera-stream works for now. I'll try to keep an eye on it, and report if I find something systematic about it. (So far I didn't do a reset of Cura Connect.)