Inger, Suzanne, thanks for the fast helpful response!
1. Yes, so far I used the Update Library function only with the library preview window open. I shall try to close it and will report in this thread what happened. (it may take a few days)
2. I noticed that when the library problem occurs, then unwanted fragments of code from open pages (like index.html) were copied into the library code. Does this info help?
3. I shall be pleased to try out the announced new build. If it solves the problem, then there would be no need to get a support ticket, right?
Please submit a support ticket so we can get needed screenshots, files, and other info from you. I have not been able to reproduce the problem yet so I think it has a lot to do with what files are open, preview on/off, possibly word wrap, and whether the *.lbi files were edited outside of the editor, etc. I've also identified a possible issue if you pasted code from the web into the library file (mismatched line endings), but haven't really reproduced that either. Thanks for helping track this down.