I could reproduce the error using the 1 sec image as mentioned above.

1)  setup camera collection as expose 1 sec with 50 count, with filter/gain/offset as needed, give name (Lseq1sec50.....esq)
2)  setup schedule on target (Dubhe) with no track, no focus, no align, no guide using sequence from (1).  setup 20 targets with names like (Dub-1, Dub-2, Dub-3, ... as repeats)

a)  When it runs I get to Dubhe-7 set (about 350-400 images collected).
b)  Fail shows up as "invalid" and issue highlighted above.  Image attached shows sequence in middle of fail.  Second image shows (a) the list of files in D-7 set and you will see gets multiple files named file_xxxxxx_039.fits (this was the beginning of the fail).  I "reset" the camera without stopping the sequence or capture because failed on download of image. (these are the xxxx_040.fits files).  after it gets all the way through the 50 files in the set, the sequencer notes complete and all other items are "invalid", so does shutdown process and closes all items and stops.
c)  Attempt to "restart/reconnect" ekos without closing application will result in eventual kstars application crash. I restarted ekos to check the camera and crashed.  Restart of kstars, restart of ekos after crash will work OK -  this is what I normally do when I get invalid as other UI features are broken if you do not restart.
d)  attached are esl and esq files.