Your comments

what exactly did you change in the windows installation that is working ok now? is it easy to implement on all Filerun users' computers?

mbstring is active on this server:

You are right, there is no error of that type on the demo server.

I also get this mysql error - SQLSTATE[HY000]: General error: 1267 Illegal mix of collations (utf8mb4_unicode_ci,IMPLICIT) and (cp1251_general_ci,COERCIBLE) for operation '='

I figured it out - it's the capital letters - if there are more than 5 capital letters in the file name - the missing id appears, if less, the file becomes viewable. Weird!

I think it's something to do with the file names. I changed the names and the files become viewable. 

I changed the file viewer to Zoho, but it was reporting a similar error, so it is not from the viewer plugin.

But I can not really figure out what exactly in the name breaks the code. Any special characters like dashes, dots, capital letters?

I need to add that the files are loaded from a base folder which is CHMOD 755, I have uploaded them via ftp. May be change the CHMOD?

Hi, in the access log I can see a track of my actions with my IP, but also an IP of Microsoft acessing - I did try to open the 3 documents again and a second document is missing weblink id now. Is it possible that Microsoft is breaking the links?