The transfer of a self-created GPX track to my Edge 1030 Plus always fails. I tried it with BaseCamp, Connect and USB cable, the file was always deleted after the restart

you put ir into the New files directory?

There is a limit to the number of Gpx files which can be stored. If the limit is exceeded ( I think 200) then new gps cannot be stored. Then remove older one and try again (when doing manually place the file into New files directory

The Edge series is limited to a maximum of files stored in the directories

100 Courses.fit

200 Favorites  (POI)

The Garmins are picky about the organization of the GPX files. Not all will work.

Basically, your file isn’t correct.

You could try converting your GPX file using something like:

https://www.gpsvisualizer.com/

You could also compare your file with files known to work (try Garmin or RWGPS.

The Garmins deletes the file so it won’t get stuck trying to convert bad files repeatedly.