I think I get pretty much the same issue:

dsync(support): Panic: file mailbox-attribute.c: line 360 (mailbox_attribute_get_stream): assertion failed: (value_r->value != NULL || value_r->value_stream != NULL)
dsync(support): Error: Raw backtrace: /usr/lib64/dovecot/libdovecot.so.0(+0xc9e06) [0x7fba8a348e06] -> /usr/lib64/dovecot/libdovecot.so.0(default_fatal_handler+0x2a) [0x7fba8a348e4a] -> /usr/lib64/dovecot/libdovecot.so.0(i_fatal+0) [0x7fba8a2bd813] -> /usr/lib64/dovecot/libdovecot-storage.so.0(+0x597fc) [0x7fba8a64f7fc] -> dsync(dsync_mailbox_import_attribute+0x3a) [0x55d72a8ac37a] -> dsync(dsync_brain_sync_mails+0x2ef) [0x55d72a8a7e9f] -> dsync(dsync_brain_run+0x2b0) [0x55d72a8a3690] -> dsync(+0x29f5c) [0x55d72a88af5c] -> dsync(+0x2bd97) [0x55d72a88cd97] -> dsync(+0x2c878) [0x55d72a88d878] -> dsync(doveadm_mail_try_run+0x205) [0x55d72a88e1f5] -> dsync(main+0x475) [0x55d72a87de85] -> /lib64/libc.so.6(__libc_start_main+0xf5) [0x7fba89c8d725] -> dsync(_start+0x29) [0x55d72a87e009]

with command:

doveadm backup -x "virtual/*" -x "shared/*" -u support sdbox:/srv/dovecot/support/sdbox

with version 2.3.2.1