Missing horizontal frame

I added a frame at the end of a line, see attached file. (second bottom line, this page is an extract of a larger score). After a number of additional save versions, this "frame" can't be selected any longer. Other frames elsewhere in the score work fine. This missing frame still takes up space on the line and I can insert additional frames from the bar in front, but I can't delete this frame nor edit it.

Any suggestions?