Thank you, wilbert!
I just did further tests and found the following:
The ConvertCharPtr() function don't have a problem itself, it crashes at the call of MemoryStringLength() because a string pointer of an empty string was given a parameter. It would be easy to catch this, if including a test before...
But as this would be an important speed impact, such a test must be done before calling the ConvertCharPtr() function and only if really needed.
Using this "workaround" is solving my problem.
But I searched further: as the crash only happened, when switching between the "single search" (comparing only specific data-fields) and my implementation of a "full-text search" and back to a "single search", I noticed, that my Map of (e.g. Cities) >75,000 elements is increased by one element after the full-text search. And this one additional element contains only empty structure fields, finally causing the crash when calling the ConvertCharsPtr() function on an empty search string...
I don't find a mistake in my code, the only peculiarity in my full-text search is, that's not only a simple loop through all map elements (which the "single search" is), but also use a pair of Push/PopMapPosition in the meantime. But from my point of view this shouldn't increase the map itself. (It's increased by one element also only one time, even if calling the full-text search several times.)
I need to try, if I could isolate this problem in a small test code. Maybe I've found a bug in some special circumtances...
About your question about the full-text search:
It's just my implementation of it, "collecting" the content of all available data fields for one element (city, country, etc.) and the comparing this (probably large) string with one or several searchterms, according to selected search-mode "contains all", "contains one", "contains none...", etc. As it's working well, I didn't thought about any change to it...
Beside of that, the single-search is comparing each individual data field, if there is a search term for it... (again according to selected search-mode, this time different for text and numeric fields).
If I'm remembering right, I am oriented towards office programs (e.g. MS Excel) and their search possibilities...