Premiere Pro export error -1609629695

This issue cant just be related to text over png.  I have a project that wont render with the same error code. I thought this was only happening in the most recent version, but after rolling back to 25.1 - where I had no problems - and then forward to 25.2.1 i'm getting the error again).  I have manged to successfully export once - by rendering previews (sequence set to quicktime 422HQ) and then rendering using previews to the same settings - and then using ME to encode to h.264.  What a faff.  WHen the render fails it appears to take the graphics pathway with it, because I need to restart premiere to get any playback.
THings I tried

1.The sequence contains text over a mogrt, which I nested and still got the error.
2. removing all those nests: sequence rendrered OK
3. Nesting the nests -still failed (at a timecode that didnt appear to have a title?!)
4. tried again to export without the sequences (recreated the sequence-free timeline): rendered ok (wasnt a fluke)
5. Removed all but the few around where the render failed:  still failed
6. Cleaned up the nested sequences so the end of the clips were at the same point (I also set one of the clips to 50% speed instead of the 49.95 that it had somehow been set to previously): success!

So not sure if this helps you isoloate the issue, after and before screengrabs below.