OK, I think I have this nailed now. The devil is in the details, as usual. I have adapted the flags for the GetText method so that only the relevant text strings are returned, i.e. a text line that spans multiple lines will be retrieved as one single line. Some irrelevant objects are ignored, but some have to be retrieved and used to not touch the text that follows. This is all pretty tricky work, but the code below does the trick without crashing, at least in some of the documents I have tested it on. It processes one single flow, so if you have documents with more than one flow, you will have to repeat the script for each flow - after first placing the text cursor at the start of that flow.
The script is getting a little too long to post here, so if you drop me an e-mail to jang at jang dot nl I will send the jsx file to you. That saves you copying and pasting and possible mishaps due to the conversion from jsx to web to jsx.
Ciao
Jang