Finally....everything works like a charm. What I did...? Yes I did set up Serv-U to use(listen to) the standard PORT 21, forwarded ONLY port 21 in my router and did not setup a passive port range in Serv-U...thatīs all.
I guess there is a limitation in the router software/combined with Serv-U/ that not can handle other ports than 21 for ftp, maybe there is a possibility to define "complex" filter rules that can handle other port-range but this might be a new project for next month
Many ISPīs donīt like "private ftp-servers" running in there network so they might supervise this commom port for ftp-ing.
I might mail netgearīs support about this matter
Cheers
IB