[Dovecot] doveadm backup panic

[Gedalya]

using latest auto build didn't help.
this happens only with a specific account.

# doveadm -o imapc_user=----- at domain.com -o imapc_password=---- backup 
-u =----- at domain.com -R imapc:
dsync(---- at domain.com): Panic: pool_data_stack_realloc(): stack frame 
changed
dsync(---- at domain.com): Error: Raw backtrace: 
/usr/lib/dovecot/libdovecot.so.0(+0x4209a) [0xb762b09a] -> 
/usr/lib/dovecot/libdovecot.so.0(default_fatal_handler+0x41) 
[0xb762b1a1] -> /usr/lib/dovecot/libdovecot.so.0(i_fatal+0) [0xb75fd99e] 
-> /usr/lib/dovecot/libdovecot.so.0(+0x547ce) [0xb763d7ce] -> 
/usr/lib/dovecot/libdovecot.so.0(+0x3ebf9) [0xb7627bf9] -> 
/usr/lib/dovecot/libdovecot.so.0(buffer_get_space_unsafe+0x78) 
[0xb7628658] -> 
/usr/lib/dovecot/libdovecot.so.0(buffer_append_space_unsafe+0x2a) 
[0xb762875a] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x60005) 
[0xb76dc005] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x62aa9) 
[0xb76deaa9] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x66e61) 
[0xb76e2e61] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(imapc_connection_input_pending+0x14d) 
[0xb76e3a3d] -> /usr/lib/dovecot/libdovecot-storage.so.0(+0x67aac) 
[0xb76e3aac] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_call_io+0x44) 
[0xb763a034] -> 
/usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run+0xce) [0xb763ae8e] 
-> /usr/lib/dovecot/libdovecot.so.0(io_loop_run+0x40) [0xb76399d0] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(imapc_client_run+0xa7) 
[0xb76e0297] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(imapc_storage_run+0x26) 
[0xb76df1e6] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(imapc_mailbox_sync_init+0x6de) 
[0xb76ddfde] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mailbox_sync_init+0x3b) 
[0xb76f468b] -> 
/usr/lib/dovecot/libdovecot-storage.so.0(mailbox_sync+0x3c) [0xb76f47cc] 
-> doveadm() [0x8072510] -> doveadm() [0x8072789] -> doveadm() 
[0x80729b2] -> doveadm(dsync_worker_msg_iter_next+0x29) [0x8070a69] -> 
doveadm() [0x806b6e1] -> doveadm() [0x806b736] -> 
doveadm(dsync_brain_msg_sync_more+0x4cb) [0x806bc7b] -> 
doveadm(dsync_brain_sync+0x3bb) [0x806a3ab]
Aborted