Scrobbling tracks to Last.FM with WinAMP 5.5 (for Windows) is apparently broken if you happen to use the old "stand-alone" Audioscrobbler plugin v1.1.10 for WinAMP as provided by Last.FM.
Fortunately, I found the Winamp 5.5 Scrobbling Problem thread in the forums with 2 solutions. For me, installing the stand-alone Audioscrobbler plugin DLL provided by DoctorO helped. Just download the hacked
http://nunzioweb.com/daz/AudioScrobbler/5_5/gen_audioscrobbler.dll1), quit WinAMP and replace the DLL (at your own risk, of course). Big thanks to DoctorO.