Stanislav,
We have mix of Computers with various versions of MS Office (2007 and 2010)
and LO (3.4.2) in our company installed. Now I make some steps, which
eliminate using mix MS Office and LO upon one shared document in workgroup.
So my offer to participate in debugging XML filters on LO is more guided by
own interests than actual company needs....
I know of cource about option of using XLS file format instead of XLSX....

We are using LO 3.4.2. on Win XP and Win 7, x86 and x64 variants. Problems
not depend on that however.
So, is any interest in inspecting these files in LO developers comunity?
Or is this mailing list wrong place for that type of problems?
To answer your question, you can send the files privately to one of the
monitors (me for example) to be looked at. Or you can post them and send
a link to the list, if there is nothing sensitive in a typical file, the
second is better. If neither is a good option, then could you construct
a test file for review? Attachments are stripped off any email.
Is there any specific error that happens, such as formula not properly
calculating?
After the problem has been identified, we can decide if a bug report is
necessary