I've been following a tutorial by Daniel Kreuter on Youtube (https://www.youtube.com/watch?v=wdsY4BtlcpQ), modelling a body using a mirrored surface. I thought I had followed his actions fairly accurately, but I'm getting a weird seam on the mirrored plane in my model when I apply a multiresolution subdivide. Oddly enough, it doesn't appear when I use the subdivision surface modifier.
I'm not really sure where to start looking for problems. This question has an "answer" which seams (see what I did there?) to have the same problem I'm having, with no solution. This forum post looks like they have the same problem. One of their suggestions is to move the mirror modifier above the subdivide, as the modifiers are applied sequentially. Unfortunately, that gives me the error "Cannot move a modifier requiring original data". The other suggested fixes, checking clipping (it was already checked) and increasing the merge limit, haven't worked.
Another thing I've checked, based on what I've seen on the web, is whether faces have been inserted along the mirror plane by accident. Sadly, that's not the solution either, nor is it due to duplicated vertices. What should I try next?