just to report on my problem.
in this folder /var/register/system/net/router/PAT
there is a folder for each rule in the “virtual servers”
the rules had reach the number 100 (although there were big gaps between the numbers)
Every new rule that was created from the web interface in the virtual servers tabs, was overwriting the contents of the “100” folder.
so, i moved the folder to lower numbers and then the creation of new rules was working again.
Disclaimer: i am not sure if this is the correct action to resolve this problem, but this worked for me.