Peter,
thanks for the screenshot and .vf file.
I gather the screenshot is from the "unified" video which prompted no error message later
and the .vf file is from the one that prompted the request.
Did/could you test just once more adding a marker to the "ununified" timeline between that 19 and 26 minute mark in
VMS Platinum?
In Metadata options see that "Save project markers in rendered media file" is checked.
In Customized template see that "Insert I-frames at markers" is checked as well.
Re-render.¹
(You may need to press Load Markers in DVD A Studio after inserting media.)
See if DVD A Studio 5 proceeds without messages then.
Seems certain that the "2nd video" on the far right is the reason.
I cannot say why, but some posts in
this discussion
elsewhere may or may not give a clue.
¹) If you want to test re-rendering only the audio with the marker,
this discussion
might be interesting. Have not tried myself to see if it'd work with Studio versions.