Hi,
I’m trying to back up a movie of 155mins. However Decrypter says the parameters are incorrect.
Any ideas?
Hi and welcome to the forum 
Probably there is a new protection that the old DVD Decrypter is not able to manage. You need a more recent and updated solution like DVDFab Decrypter (free) or AnyDVD (not free).
Other information you should give for help.
What movie ?
What kind of disc is the movie now on?
Example : commercial DVD
Size of movie on disc. x.x GB
What are you backing up to ?
HDD or disc ?
What type of disc?
Your problem is probably because You are trying to put a DVD9 on a DVD5 which will not work with DVD Decrypter you need a shrink app. that can compress a 9 to a 5 like DVDShrink.