I think I know why. Shortly after getting SGP, I ran into some guiding problems. In an attempt to get to the bottom of these, I recall that I did not connect to my mount through SGP when shooting the Pelican. That would explain why there is no data in the RA and DEC boxes for the Pelican whereas there is for subsequent images taken.
I have now been able to get PS2 to work. I Blind Solved using Astrometry.net, and this gave me values that I could put in for Angle and for Scale. When I entered these into the boxes, PS2 did a rapid solve. Playing around with it, it seemed that I had to enter something into Scale or else PS2 would not solve (and report '0 x 0' in the 'arcminutes' box). Leaving Scale at '0' reproduced the Run-time error. Angle was less of an issue, and I seemed able to leave this at '0' (in fact the image was at 169 degrees). With my set up, entering a Scale of '2' seemed to work consistently. For images where I had no RA and DEC values, entering the NGC catalogue number also worked in lieu of entering actual RA and DEC.
So, assuming that I am now using PS2 correctly, my problem is solved - it was down to user error.