Turns out the issue was in fact trivial and somewhat due to my lack of experience with python and misleading error message.
The COM object raw = win32com.client.Dispatch("MyLib.MyClass") is used to open proprietary files in a loop. To solve the issue one must “clean-up” the object before next iteration. This is done either by
del raw
or
raw = None.
That completely solves the issue. It has absolutely nothing to do with dates or date formating. So Peter Brittain was probably right that this file limit was reached.