alright, i managed to capture the problem on screenshots. however, it has nothing to do with pausing mediamonkey. i started mediamonkey, and started playing music right away.
the first song was registered by last.fm, and showed up as "now playing":
after the first song successfully scrobbled, the second song appeared as "now playing":
however, the third song did not appear anymore, and the second song now mysteriously disappeared from last.fm...:
... while i am actually playing the third song:
in the end i only have the first song that successfully scrobbled:
from this moment on, no songs will scrobble anymore. not when I pause for a bit, not when I pause for a long time (long enough to get rid of the "just listened" from last.fm), not when i force the next song. only when i restart mediamonkey again it will start scrobbling again.
i had this problem with the current version of mediamonkey (220.127.116.113), and with both versions of the last.fm scrobbler plugin (1.0 and 1.1) on a windows 7 x64 machine. since i seem to be the only one with problems, i fear the problem might be on my side, but i have no clue where to look for answers.
for the record: i restarted mediamonkey now, and started playing, and everything seems fine now. i still haven't been able to pinpoint the source or trigger of the problem.