The message come from IIS. I suspect that the system is trying to pass parameters to the htm page, as per the following link: www.ert.gr/keep/forma-ipobolis/contacts/....e.htm?mosmsg=%CE%95

Probably passing parameters to an htm page is not supported (?).

I tried to disable SEF for com_contacts to no effect!

Well if you are using IIS, are you sure it cannot be some wrong rewrite rule somewhere?

Also, under what conditions it was working? What version did you have before? Will it work if you revert back to that version now?

It had not been tested with a previous version of JoomSEF before, hence we don't know whether it works with a previous version.

The rewrite rules used are the default Joomla ones...

Well, what rewrite engine are you using for IIS?
Maybe it does not support all directives from Joomla!, so you may need to adjust them.

IIS6, PHP5, Helicon ISAPI_REWRITE latest build.

Ok, good choice. Do you use Lite or paid version?

Can you also post here your rewrite rules then?