Meanwhile I asked me if it is need also to disable K2 login (K2-User).
Yes, you should always be using CB login and registration. Using anything else will cause CB to go out to sync. K2 has a CB integration, which should be enabled from within K2 so it points URLs to CB.
You speak continuosly about "maximum error reporting", but where is this option?
It's in Joomlas global configuration. Please see Joomla documentation on its usage and standard Joomla debugging procedures.