Hi All, we don't have quite enough information from you, a build number would help, but without it, here are our suggestions.
First of all, please check if you are using the latest update! If you are using the latest update and still having this problem, collect FastLogger files for us. The instructions for creating those files can be found here:
http://helpcenter.graphisoft.com/?p=63116
It is the best to save the FastLoggers after rendering action in ARCHICAD 20.
Kind regards,