Well, nope, looks like I was right:
After 4095 letters a String is full, so if either my "BEFORE" or "AFTER" is full, everything that comes after will be ignored and therefore deleted frown

The limit of 4000 is described in my A7.5 manual, newer versions of the engine support more, but still not unlimited
(http://manual.conitec.net/file_str_read.htm)

I need to somehow divide everything up into multiple reading-parts and then put them back together.
I will post a solution if I find one that works good laugh