Synchronization problem in nt_send_process?

In XEmacs 21.2 (beta37) "Pan" [Lucid] (i586-pc-win32) of Wed Nov 15 2000 on SLOTH configured using `configure UNKNOWN' For a while now I've been having Emacs lock up with 0% CPU usage from time to time when trying to copy a somewhat sizable messages to a nnimap folder. Looks like a deadlock of some sorts: Gleb NTDLL! 77f6828b() USER32! 77e7ab66() USER32! 77e7aaba() mswindows_need_event(int 1) line 1483 + 33 bytes emacs_mswindows_next_event(Lisp_Event * 0x02865ac4) line 3278 + 7 bytes event_stream_next_event(Lisp_Event * 0x02865ac4) line 499 + 13 bytes next_event_internal(long 42359492, int 0) line 1958 + 9 bytes Faccept_process_output(long 28360708, long 1, long 2001) line 2610 + 11 bytes nt_send_process(long 35274496, lstream * 0x02429b80) line 1085 + 26 bytes send_process(long 35274496, long 42486784, const unsigned char * 0x00000000, int 1, int 24318) line 1001 + 23 bytes Fprocess_send_region(long 35274496, long 3, long 48639, long 42486784) line 1109 + 26 bytes Ffuncall(int 4, long * 0x0082e620) line 3528 + 168 bytes execute_optimized_program(const unsigned char * 0x021a4c10, int 4, long * 0x01d3b510) line 746 + 16 bytes funcall_compiled_function(long 35927980, int 2, long * 0x0082e900) line 518 + 53 bytes Ffuncall(int 3, long * 0x0082e8fc) line 3563 + 17 bytes execute_optimized_program(const unsigned char * 0x01d3a510, int 4, long * 0x01d3abf0) line 746 + 16 bytes funcall_compiled_function(long 30666640, int 1, long * 0x0082ebd4) line 518 + 53 bytes Ffuncall(int 2, long * 0x0082ebd0) line 3563 + 17 bytes execute_optimized_program(const unsigned char * 0x0286fe90, int 6, long * 0x01f05390) line 746 + 16 bytes funcall_compiled_function(long 35927868, int 5, long * 0x0082eeb4) line 518 + 53 bytes Ffuncall(int 6, long * 0x0082eeb0) line 3563 + 17 bytes execute_optimized_program(const unsigned char * 0x0286ff90, int 6, long * 0x022aa190) line 746 + 16 bytes funcall_compiled_function(long 35305812, int 3, long * 0x0082f194) line 518 + 53 bytes Ffuncall(int 4, long * 0x0082f190) line 3563 + 17 bytes execute_optimized_program(const unsigned char * 0x0286ff10, int 4, long * 0x01cd6d90) line 746 + 16 bytes funcall_compiled_function(long 35984344, int 4, long * 0x0082f46c) line 518 + 53 bytes Ffuncall(int 5, long * 0x0082f468) line 3563 + 17 bytes execute_optimized_program(const unsigned char * 0x0285c810, int 10, long * 0x02207410) line 746 + 16 bytes funcall_compiled_function(long 36964412, int 4, long * 0x0082f75c) line 518 + 53 bytes Ffuncall(int 5, long * 0x0082f758) line 3563 + 17 bytes execute_optimized_program(const unsigned char * 0x0285be10, int 5, long * 0x022fe490) line 746 + 16 bytes funcall_compiled_function(long 36964440, int 1, long * 0x0082fa34) line 518 + 53 bytes Ffuncall(int 2, long * 0x0082fa30) line 3563 + 17 bytes Fcall_interactively(long 35755860, long 28360708, long 28360708) line 940 + 22 bytes Fcommand_execute(long 35755860, long 28360708, long 28360708) line 2970 + 17 bytes execute_command_event(command_builder * 0x01d15c80, long 31425264) line 3833 + 25 bytes Fdispatch_event(long 31425264) line 4124 + 70 bytes Fcommand_loop_1() line 583 + 9 bytes command_loop_1(long 28360708) line 495 condition_case_1(long 28360804, long (long)* 0x01050b66 command_loop_1(long), long 28360708, long (long, long)* 0x01050580 cmd_error(long, long), long 28360708) line 1651 + 7 bytes command_loop_3() line 256 + 35 bytes command_loop_2(long 28360708) line 269 internal_catch(long 28440700, long (long)* 0x010506d0 command_loop_2(long), long 28360708, int * volatile 0x00000000) line 1317 + 7 bytes initial_command_loop(long 28360708) line 305 + 25 bytes STACK_TRACE_EYE_CATCHER(int 1, char * * 0x01b43f30, char * * 0x01d2cf00, int 0) line 2233 main(int 1, char * * 0x01b43f30, char * * 0x01d2cf00) line 2666 XEMACS! mainCRTStartup + 180 bytes _start() line 169 KERNEL32! 77f1