Strange. I've not seen this before, and I couldn't reproduce after trying twice 
(Mobbler 0.3.1 dev, Nokia N95 8GB v30.0.018 / 23-07-08).

That error is coming from Last.fm, from the iPodScrobbler FAQ:

===
Q: I keep getting this error: "FAILED Plugin bug: Not all request variables are set 
- no POST parameters." What gives?
A: This is actually some bug on Last.fm's part, that I could not resolve. The 
requests are definitely correct, but sometimes they still fail. I assume it's due to 
some of their anti-spam features (or maybe server overload), I haven't been able to 
get a reply from them. They don't really like bulk upload tools, it's against the 
concept of their site (online listening, uploading one song at a time). 
They sort of tolerate it because they have to, too many people use iPods after all. 
I have added a fix for this problem, iPodScrobbler now retries the request up to 3 
times. This is usually sufficient to get it through to Last.fm's servers. If this 
doesn't help and you still get that error, you can either try to increase the number 
of retries in the script (there is a variable near the top) or you can use the --
timestamp option to restart the upload again just before that last song that did not 
get submitted (it's cumbersome I know).
===

http://www.hoc.net/~mike/source/iPodScrobbler/README_Quickstart.html

 Issue 158  has been merged into this issue.

From looking around using google, it looks like a lot of scrobblers have this problem
when trying to submit many tracks.  It's been suggested that the Last.fm servers are
not reading the complete posted form or the form is being truncated on the way to
Last.fm, or something like that.

Most people seem to report that it starts working again after a few re-trys.

One thing that Mobbler could do is only ever send one track at a time to Last.fm. 
This would be quite inefficient, but would solve the problem.

from my experience, this problems happens when the offline queue goes into double 
digits, so if mobbler sends 9 tracks maximum at a time, the problem should disappear, 
and it wouldn't be that inefficient.

I ran into this issue this morning (Nokia N96, latest firmware, Mobbler 0.04(5)). I
don't know about the double digits assertion, I had a little over 350 tracks queued
(listen during the day at work and then upload at home on WiFi) and Mobbler
successfully got through the first 350 in batches of 50, but started reporting the
"FAILED Plugin" message while trying to submit the last few tracks.

I can confirm this issue with Mobbler 0.6(5124) on a Nokia 5320 - not more than 11
Tracks are being submitted at a time, the others get lost, although mostly without an
error message. I've seen the message mentioned above only once or twice.

Now, this is a bit strange. For a few times I could scrobble 60 to 75 queued tracks
at once without a problem. A few minutes ago, I tried to scrobble 80 tracks, and got
no error message, but only the oldest 10 of those tracks show up in my lastfm-profile.

infrequently i have that problem but after a couple of trying to send the queue when 
online all tracks was scrobbled to last.fm not only oldest 10.

Lately I tend to believe that scrobbling works more reliably when using a 3G/UMTS
instead of a 2G/GSM/GPRS connection. But - and this might be another issue - the
display of the queued/scrobbled numbers doesn't change on its own sometimes when
online. The number of queued tracks switches to zero only when I use (i.e. open and
close) the Mobble menu or when I go offline.

(The scrobbled/queued thing is really another issue, but it's very easy to fix, 
fixed in ra7d5de21cf)

My device has the same problem too but I found trying another connection may work.

My device is Nokia 5800 XpressMusic. When mobbler has queued scrobbles, switch to
online mode. It asked me which connection type. If I choose Wi-Fi network, mobbler
failed to send scrobbles sometimes. Then switch to offline mode and switch to online
mode again. This time I choose WAP network (the data plan of operator), mobbler
always send queued scrobbles without any problems.

i confirm this issue with Mobbler 0.07(6187) on Nokia 5320
i had about 75 tracks in queue and when online mode an error message "FAILED Plugin 
bug: Not all requests variable are set - no POST parameters." appeared two or three 
times then all queued tracks was scrobbled.