export problem - seems related to captions

I have a new issue with 23.5. When I attempt an h264 export, the first part of the video exports fine, but then it goes to black. I can see where the colored render bar ends in the export window. Its ending point shifts with each export attempt, so it doesn't seem related to a cut point or clip. The video was also very, very slow to render and then the render didn't stick. When I turn off the captions track, the export works fine. When I turn the captions track on again, the issue described above happens again. I have tried creating new sequences and re-importing the .srt, but that didn't fix it. I have tried thanging the renderer to software only. Didn't fix the issue.