Sorry for asking but have you nptice that I am running in MM Framework?
Yes - it does not matter. (I have not used MM, because we use our own performance and features optimized framework)

... and then trying to preview, FoxPro freeses out and I can't do anything.
Write

DEBUG
SUSPEND

statements before and after the

Report form ... Preview

(also notice that without IN SCREEN clause, Report form statement will do the preview in your debugger window!!!)

... or if reporting is class-wraped, e.g. you use an object method to preview the report, subclass the objects class and in its ReportPreview method (or however it is named) write code like the following

DEBUG
SUSPEND  && to stop program execution before the report is previewed
DODEFAULT() && here in brackets you may need to enter the methods parameters (if you have parameter statement on the first line)
SUSPEND  && to stop program execution after the report is previewed

... and then I get something about stack or something and everything in the debugger gone and I can't even see where the error occured.
check the

program(-1)

value in this moment. If it is 0 (zero) it means that at the moment processing is on the READ EVENTS statement (the ReportPreview method has finished its execution)

How's Varna, everything OK out there?
Yep, it's fine here... waiting forward for warmer weather and more VFP projects
(Have you been to owr small town at the Black Sea coast?)