The Htmemb (membrane height) input keyword is not active in the PBEQ module, although it is listed as an input parameter.
Rather the HEAD keyword is active instead. I tried the HEAD keyword with solve, and it works. Htmemb does not work.
The reason for this appears to be the following.
In the file ~c35b2/source/misc/pbeq.srs (or ~c34b2/source/misc/pbeq.src), there is a line
c Stuff for Membrane
c thickness of headgroup region
HTMEMB = GTRMF(COMLYN,COMLEN,'HEAD',ZERO)
which probably should be modified to something like
HTMEMB = GTRMF(COMLYN,COMLEN,'HTME',ZERO)