Is it possible that the CS-JS-file for the extension library dialog has changed between 8.5.2 and 8.5.3 library?
For performance issues we use the expired headers (so that the browser caches the JavaScript-files for some days). We have switched to Domino 8.5.3 (with the corresponding extension library).
I think that the clients which had the problems had an old version for the JavaScript-File in the local cache.
Could that be the problem? Is it possible to rename the files in future releases (like the dojo files which have different directories so the browser couldn't keep them in the cache)?