Please see here,
http://forum.mikrotik.com/t/v5-6-released/49110/1
Regarding the L7 problem:
For some time it was known that some specific layer7 filters in combination with specific traffic may cause router to crash.
We were able to narrow down the problem and introduced a first version of the fix - as soon as specific conditions for possible crash was met firewall will add a log entry to inform about situation and will avoid the crash.
The current “fix” actually contained a bug, so it produces unnecessary log entries.
We are still working on final solution, in fact it is already known that version 5.7 will have more precise fix that will significantly reduce blocked regexps as we narrow the problem even more (thanks to your reports)