It causing with mail.ru, they have option to create separate password for outer app. There is a log:
Mailspring Version: 1.10.8-8ea2a61f
Platform: linux
Account State: sync_error
Account Provider: imap
IMAP Server: imap.mail.ru
SMTP Server: smtp.mail.ru
1172644 [2023-08-23 13:27:43.369] [metadata] [info] Metadata delta stream starting…
1172644 [2023-08-23 13:27:46.369] [background] [info] Marking all folders as busy
1172644 [2023-08-23 13:27:46.369] [background] [info] Syncing folder list…
1172644 [2023-08-23 13:27:46.876] [background] [critical]
*** Mailspring Sync
*** A C++ exception occurred during program execution:
*** UNIQUE constraint failed: Folder.id
1172644 [2023-08-23 13:27:46.930] [background] [critical] *** Stack trace (line numbers are approximate):
*** ??:? exceptions::logCurrentExceptionWithStackTrace()
*** ??:? runBackgroundSyncWorker()
*** main.cpp:? main::{lambda()#3}::operator()() const
*** main.cpp:? _Bind_simple::operator()()
*** main.cpp:? thread::_Impl::_M_run()
*** thread.o:? execute_native_thread_routine()
1172678 [2023-08-23 13:27:47.310] [main] [info] Identity created at 1582038163 - using ID Schema 1
1172678 [2023-08-23 13:27:47.311] [main] [info] ------------- Starting Sync (L***n@mail.ru) ---------------
1172678 [2023-08-23 13:27:47.316] [metadata] [info] Metadata delta stream starting…
1172678 [2023-08-23 13:27:50.316] [background] [info] Marking all folders as busy
1172678 [2023-08-23 13:27:50.316] [background] [info] Syncing folder list…
1172678 [2023-08-23 13:27:50.860] [background] [critical]
*** Mailspring Sync
*** A C++ exception occurred during program execution:
*** UNIQUE constraint failed: Folder.id
1172678 [2023-08-23 13:27:50.916] [background] [critical] *** Stack trace (line numbers are approximate):
*** ??:? exceptions::logCurrentExceptionWithStackTrace()
*** ??:? runBackgroundSyncWorker()
*** main.cpp:? main::{lambda()#3}::operator()() const
*** main.cpp:? _Bind_simple::operator()()
*** main.cpp:? thread::_Impl::_M_run()
*** thread.o:? execute_native_thread_routine()
1172730 [2023-08-23 13:27:51.293] [main] [info] Identity created at 1582038163 - using ID Schema 1
1172730 [2023-08-23 13:27:51.294] [main] [info] ------------- Starting Sync (L***n@mail.ru) ---------------
1172730 [2023-08-23 13:27:51.299] [metadata] [info] Metadata delta stream starting…
1172730 [2023-08-23 13:27:54.299] [background] [info] Marking all folders as busy
1172730 [2023-08-23 13:27:54.299] [background] [info] Syncing folder list…
1172730 [2023-08-23 13:27:54.753] [background] [critical]
*** Mailspring Sync
*** A C++ exception occurred during program execution:
*** UNIQUE constraint failed: Folder.id
1172730 [2023-08-23 13:27:54.808] [background] [critical] *** Stack trace (line numbers are approximate):
*** ??:? exceptions::logCurrentExceptionWithStackTrace()
*** ??:? runBackgroundSyncWorker()
*** main.cpp:? main::{lambda()#3}::operator()() const
*** main.cpp:? _Bind_simple::operator()()
*** main.cpp:? thread::_Impl::_M_run()
*** thread.o:? execute_native_thread_routine()