Dangerous is a relative term. It could be something like dirty fluid which, which can be remedied with a fluid exchange and, probably, a new filter. OR it could be a failed solenoid, which could lead to erratic shifting or dropping out of gear or whatnot which absolutely COULD be dangerous. OR it could be an indication that your radiator leaked fluid into the transmission and your van is about to catastrophically fail, making it worthless.
All that said, asking that particular question in that particular way suggests you are inexperienced in this particular area. So, the REAL answer is, YES, you should make an appointment with a trustworthy mechanic as soon as possible and should avoid driving it, of at all possible. You cannot (should not) ignore this code.