Your board shouldn't be appending the SID unless you're in the ACP. if it is then you probably have an incorrect cookie setting. (and going by the fact that you're being logged out I'd say it's definite)
Open a topic in the 3.0.x Support Forum and get someone to check your cookie settings.
QOTY:phpBB is free, good hosting is not. - robert (Lumpy Burgertushie)