IPFW ipfw: Table creation failed: Device busy - whats going wrong here?

[HL1234]

Hello,
I just want to do a simple thing, but can not find help what is going wrong
ipfw table 20 create type addr
ipfw table 20 list
--- table(20), set(0) ---

->OK but
ipfw table 31 create type addr
error: ipfw: Table creation failed: Device busy ?????

ipfw table 31 list
ipfw: failed to request table info: No such process

In the manpages ipfw(8)() are written "There may be up to 65535 different lookup tables."
Why can I not create ipfw table 31?

Thanks

Edit:
Don't know if that is important, but kern_securelevel is setting to "2" as I does the commands above.
Is there a ipfw command to show all tables which exist?

 

[Jeckt]

I'm not sure if this would fit your situation, but have you tried using a table name (like "example") instead of a number (31)?

Tables should show up with ipfw table all list