http://www.subsim.com/phpBB/viewtopic.php?t=51172
And let me guess, you're in an IXC or IXC/40?
To summarise the above thread, apparently changing:
IXC=NSS_Uboat9c|x2C43|275
IXC/40=NSS_Uboat9c|x2C43|275
in "Miscellaneous.cfg" to:
IXC=NSS_Uboat9c|x3007|275
IXC/40=NSS_Uboat9c|x3007|275
helps.