I get consistent "INTERNAL ERROR: Bitstream will be invalid" from mppenc 1.01j on "Mana MTV Unplugged" track 7, "Desapariciones". I ripped the track (latest EAC, secure mode) from a (possibly crappy) copy of the CD.

It happens when using --insane --ans 0 --nmt 15 --tmn 34 --ltq_gain -12 --shortthr 2 --xlevel

I get "INTERNAL ERROR: Bitstream will be invalid
frame: 13732 estimated: 10694 bits written: 10718 bits"

The resulting MPC crashes winamp plugin 0.92m

I suggest you update to 1.02 and 0.93a, and try again.

Edit: Sorry! Only just saw your post on the other thread about 1.02. No other suggestions!

OK tried and yes, 1.02 also fails.

I extracted a 7 seconds wav that trips both versions with the following switches:

--insane --tmn 34 --ltq_gain -12 --xlevel

Note that, in the internal error report, (written-estimated) seems to always be 24.

The (monkey compressed) file is around 900 KB. Where may I upload it? (if its useful for anything, that is)

Try without --xlevel.
As I've understood it's not working perfectly, and may cause different types of problems (including possible quality related).