Below you can see two outputs from my attached patch with a bitrate arround 1000 and a second arround 100.

The Line in C++ wich calculate the converge:
if(fabs(expected_bits/all_available_bits - 1.0) > 0.01 )

...and the errormessage is absolut correct! See below!

Header descriptio:
expected_bits, (int)all_available_bits, rate_factor

First the bad thing (forced bitrate was 1013):
7829297.993239 40520000 65536.000000
7829627.646038 40520000 98304.000000
7829628.142840 40520000 114688.000000
7829628.322425 40520000 122880.000000
7829628.402868 40520000 126976.000000
7829628.435996 40520000 129024.000000
7829628.471097 40520000 130048.000000
7829628.487292 40520000 130560.000000
7829628.489526 40520000 130816.000000
7829628.490026 40520000 130944.000000
7829628.491337 40520000 131008.000000
7829628.491337 40520000 131040.000000
7829628.491337 40520000 131056.000000
7829628.491337 40520000 131064.000000
7829628.491337 40520000 131068.000000
7829628.491337 40520000 131070.000000
7829628.491337 40520000 131071.000000
7829628.491337 40520000 131071.500000
7829628.491337 40520000 131071.750000
7829628.491337 40520000 131071.875000
7829628.491337 40520000 131071.937500
7829628.491337 40520000 131071.968750
7829628.491337 40520000 131071.984375
7829628.491337 40520000 131071.992188
7829628.491337 40520000 131071.996094
7829628.491337 40520000 131071.998047
7829628.491337 40520000 131071.999023
7829628.491337 40520000 131071.999512
7829628.491337 40520000 131071.999756
7829628.491337 40520000 131071.999878
7829628.491337 40520000 131071.999939
7829628.491337 40520000 131071.999969
7829628.491337 40520000 131071.999985
7829628.491337 40520000 131071.999992
7829628.491337 40520000 131071.999996
7829628.491337 40520000 131071.999998
7829628.491337 40520000 131071.999999
7829628.491337 40520000 131072.000000
7829628.491337 40520000 131072.000000
7829628.491337 40520000 131072.000000
[mpeg4 @ 0xa5af50]TKR-Error: 2pass curve failed to converge


Second the good thing (forced bitrate was much smaller then 1013, only for test):
7375465.751288 4000000 65536.000000
7376122.463696 4000000 32768.000000
7376100.880109 4000000 16384.000000
7376041.639241 4000000 8192.000000
7375878.856499 4000000 4096.000000
7375431.684153 4000000 2048.000000
7374202.972210 4000000 1024.000000
7370828.349520 4000000 512.000000
7361572.503387 4000000 256.000000
7336287.129375 4000000 128.000000
7267970.689903 4000000 64.000000
7088752.518104 4000000 32.000000
6652212.702711 4000000 16.000000
5750782.639834 4000000 8.000000
4378130.858939 4000000 4.000000
2994342.130324 4000000 2.000000
3776350.420670 4000000 3.000000
4096941.817611 4000000 3.500000
3942027.625798 4000000 3.250000
4020782.261588 4000000 3.375000
3981720.798470 4000000 3.312500
4001327.433605 4000000 3.343750
3991541.735075 4000000 3.328125
3996437.448036 4000000 3.335938
3998883.755556 4000000 3.339844
4000105.917949 4000000 3.341797
3999494.920849 4000000 3.340820
3999800.444978 4000000 3.341309
3999953.183222 4000000 3.341553
4000029.555444 4000000 3.341675
3999991.380329 4000000 3.341614
4000010.469208 4000000 3.341644
4000000.918301 4000000 3.341629
3999996.145774 4000000 3.341621
3999998.531949 4000000 3.341625
3999999.722672 4000000 3.341627
4000000.325246 4000000 3.341628
4000000.020775 4000000 3.341628
3999999.881037 4000000 3.341627
3999999.953576 4000000 3.341627
Schreibe Dateikopf...