I am currently running the 64-bit version of 2.12 (build 137) on Windows 8.1 Pro, although I've also had this happen occasionally with earlier versions and on a Windows 7 Ultimate installation, both with V3 and the deprecated implementation.
At its peak, I've been syncing 10 Google Calendars to Rainlendar, although I've been able to reproduce the issue with as little as 3. I do not have any reason to believe there is an issue unique to one of these calendars.
I have a dump available, but I am unable to link to it due to a restriction on new users. I will be sending it to the support address listed on the site.
Running the Microsoft Visual Studio 2012 debugger yields this:
Unhandled exception at 0x00007FF81967387C (ntdll.dll) in Rainlendar2.exe: 0xC0000374: A heap has been corrupted (parameters: 0x00007FF8196ACE20).
Continuing from there repeatedly yields this:
Unhandled exception at 0x00007FF80E48C47A (libical.dll) in Rainlendar2.exe: 0xC0000005: Access violation reading location 0x0000000000000008.
Any workarounds to use in the meanwhile (apart from consolidating calendars) would be much appreciated.