I have problems with KDE PIM. akonadictl start fails:
$ akonadictl start
org.kde.pim.akonadictl: Starting Akonadi Server...
org.kde.pim.akonadictl: done.
Connecting to deprecated signal QDBusConnectionInterface::serviceOwnerChanged(QString,QString,QString)
org.kde.pim.akonadiserver: Starting up the Akonadi Server...
org.kde.pim.akonadiserver: database server stopped unexpectedly
org.kde.pim.akonadiserver: Database process exited unexpectedly during initial connection!
org.kde.pim.akonadiserver: executable: "/usr/bin/mysqld"
org.kde.pim.akonadiserver: arguments: ("--defaults-file=/home/jonas/.local/share/akonadi/mysql.conf", "--datadir=/home/jonas/.local/share/akonadi/db_data/", "--socket=/run/user/1000/akonadi/mysql.socket", "--pid-file=/run/user/1000/akonadi/mysql.pid")
org.kde.pim.akonadiserver: stdout: ""
org.kde.pim.akonadiserver: stderr: "2022-05-24 11:13:41 0 [Note] /usr/bin/mysqld (server 10.7.3-MariaDB) starting as process 11259 ...\n"
org.kde.pim.akonadiserver: exit code: 6
org.kde.pim.akonadiserver: process error: "Process crashed"
org.kde.pim.akonadiserver: Shutting down AkonadiServer...
org.kde.pim.akonadicontrol: Application '/usr/bin/akonadiserver' exited normally...
$ akonadictl status
Akonadi Control: stopped
Akonadi Server: stopped
Akonadi Server Search Support: available (Remote Search, Akonadi Search Plugin)
Available Agent Types: akonadi_akonotes_resource, akonadi_archivemail_agent, akonadi_birthdays_resource, akonadi_contacts_resource, akonadi_davgroupware_resource, akonadi_etesync_resource, akonadi_ews_resource, akonadi_ewsmta_resource, akonadi_followupreminder_agent, akonadi_google_resource, akonadi_ical_resource, akonadi_icaldir_resource, akonadi_imap_resource, akonadi_indexing_agent, akonadi_knut_resource, akonadi_kolab_resource, akonadi_maildir_resource, akonadi_maildispatcher_agent, akonadi_mailfilter_agent, akonadi_mailmerge_agent, akonadi_mbox_resource, akonadi_migration_agent, akonadi_mixedmaildir_resource, akonadi_newmailnotifier_agent, akonadi_notes_agent, akonadi_notes_resource, akonadi_openxchange_resource, akonadi_pop3_resource, akonadi_sendlater_agent, akonadi_tomboynotes_resource, akonadi_unifiedmailbox_agent, akonadi_vcard_resource, akonadi_vcarddir_resource
EDIT: starting manually:
mysqld --defaults-file=/home/jonas/.local/share/akonadi/mysql.conf --datadir=/home/jonas/.local/share/akonadi/db_data/ --socket=/run/user/1000/akonadi/mysql.socket --pid-file=/run/user/1000/akonadi/mysql.pid
2022-05-24 11:20:53 0 [Note] mysqld (server 10.7.3-MariaDB) starting as process 12764 ...
Aborted (core dumped)
coredumpctl info 12764 → Stacktrace akonadi MariaDB stack trace from `coredumpctl info` · GitHub
EDITEDIT:
$HOME/.local/share/akonadi/db_data/mysql.err has many errors like this:
2022-05-24 12:15:35 0 [ERROR] InnoDB: Page [page id: space=0, page number=2] log sequence number 819501819 is in the future! Current system log sequence number 798199832.
2022-05-24 12:15:35 0 [ERROR] InnoDB: Your database may be corrupt or you may have copied the InnoDB tablespace but not the InnoDB log files. Please refer to https://mariadb.com/kb/en/library/innodb-recovery-modes/ for information about forcing recovery.
...
2022-05-24 12:21:19 0 [Note] InnoDB: 128 rollback segments are active.
2022-05-24 12:21:19 0 [Note] InnoDB: Removed temporary tablespace data file: "./ibtmp1"
2022-05-24 12:21:19 0 [Note] InnoDB: Creating shared tablespace for temporary tables
2022-05-24 12:21:19 0 [Note] InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ...
2022-05-24 12:21:19 0 [Note] InnoDB: File './ibtmp1' size is now 12 MB.
2022-05-24 12:21:19 0 [Note] InnoDB: 10.7.3 started; log sequence number 798199820; transaction id 1519280
2022-05-24 12:21:19 0 [Note] InnoDB: Loading buffer pool(s) from /home/jonas/.local/share/akonadi/db_data/ib_buffer_pool
2022-05-24 12:21:19 0x7f7ce3fff640 InnoDB: Assertion failure in file /build/mariadb/src/mariadb-10.7.3/storage/innobase/include/fut0lst.h line 128
InnoDB: Failing assertion: addr.page == FIL_NULL || addr.boffset >= FIL_PAGE_DATA
...
Is it advisable to set a recovery mode as indicated by the [ERROR] message?
EDITEDITEDIT:
Temporary Downgrade to mariadb 10.6 yields the same errors.
FINAL:
I’ve reset the databases:
mv ~/.local/share/akonadi ~/.local/share/akonadi_BACKUP
akonadictl start
# ignore some errors
akonadictl stop
# wait
akonadictl start
→ seems I have not lost any settings 
Since all my (relevant) resources are in Nextcloud/gmail/company IMAP, they are now being redownloaded.