I'll try to narrow it down further (nix the search, turn off quotas, etc.).
imaptest clients=60 user=user\%d pass=test no_tracking logout=25 copybox=Copies does it too... (no search commands)
And it happens with zero mail_plugins everywhere.
But this does not trip that assertion: imaptest clients=60 user=user\%d pass=test no_tracking logout=25 (no copy commands)
Here's a backtrace: 0 __kill + 10 1 abort + 177 2 default_fatal_finish + 68 3 i_internal_error_handler + 0 4 i_debug + 0 5 maildir_transaction_save_commit_pre + 2585 6 index_transaction_index_commit + 164 7 mail_index_transaction_commit_full + 155 8 index_transaction_commit + 131 9 mailbox_transaction_commit_get_changes + 56 10 cmd_copy + 848 (cmd-copy.c:157) 11 client_command_input + 36 (imap-client.c:687) 12 client_command_input + 416 (imap-client.c:752) 13 client_handle_input + 134 (imap-client.c:793) 14 client_input + 118 (imap-client.c:844) 15 io_loop_handler_run + 234 16 io_loop_run + 56 17 master_service_run + 27 18 main + 689 (main.c:359) 19 start + 52
FWIW I also turned on maintainer mode, but it made no difference.
What else would you like me to try?