1) You could fetch the data from the database, fix the content and import it through CalDAV/CardDAV or through the Web interface.
2) What is the full log entry? Which version of SOGo have you installed?

1.) But if I try this ways, I can not change it for others as an administrator?! The only way I see is to change the data directly in the database?
2.) We have two blocks that occur every time the user is checked. A block looks like this:
2021-08-11 14:04:35.310 sogo-tool[29290:29290] Checking folders of user employee1
2021-08-11 14:04:35.313 sogo-tool[29290:29290] Adding uh 55357 ul 56542
2021-08-11 14:04:35.313 sogo-tool[29290:29290] Adding uh 55357 ul 56544
2021-08-11 14:04:35.313 sogo-tool[29290:29290] Adding uh 55356 ul 57101
2021-08-11 14:04:35.383 sogo-tool[29290:29290] Checking folders of user employee2

for 1) a workaround exists:
You can add your SOGoSuperUser to the calendar in question with modify privileges first.
Then subsribe that calendar and change the event.
Afterwards unsubscribe and remove your SOGoSuperUser from the calendar.

Do you reproduce the second issue (Adding uh ..) with a recent nightly build?

Unfortunately I could reproduce the problem in version 5.2.0.20210929.

Here are the messages with the "PGDebugEnabled = YES;" option turned on, maybe it will help troubleshooting:
2021-09-30 14:31:39.536 sogo-tool[7889:7889] Checking folders of user employee1
2021-09-30 14:31:39.536 sogo-tool[7889:7889] PG0x0x557d78c616e0 SQL: SELECT c_path FROM sogo_folder_info WHERE c_path1 = 'Users' AND c_path2 = 'employee1'
2021-09-30 14:31:39.537 sogo-tool[7889:7889] PG0x0x557d78c616e0 SQL: SELECT c_folder_id, c_path, c_location, c_quick_location, c_acl_location, c_folder_type FROM sogo_folder_info WHERE c_path1 = 'Users' AND c_path2 = 'employee1' AND c_path3 = 'Calendar' AND c_path4 = 'personal'
2021-09-30 14:31:39.537 sogo-tool[7889:7889] PG0x0x557d78c616e0 SQL: SELECT c_name, c_content FROM sogo_store WHERE c_folder_id = 13 AND (c_deleted != 1 OR c_deleted IS NULL)
2021-09-30 14:31:39.539 sogo-tool[7889:7889] Adding uh 55357 ul 56542
2021-09-30 14:31:39.539 sogo-tool[7889:7889] Adding uh 55357 ul 56544
2021-09-30 14:31:39.539 sogo-tool[7889:7889] Adding uh 55356 ul 57101
2021-09-30 14:31:39.579 sogo-tool[7889:7889] PG0x0x557d78c616e0 SQL: SELECT c_folder_id, c_path, c_location, c_quick_location, c_acl_location, c_folder_type FROM sogo_folder_info WHERE c_path1 = 'Users' AND c_path2 = 'employee1' AND c_path3 = 'Contacts' AND c_path4 = 'personal'
2021-09-30 14:31:39.579 sogo-tool[7889:7889] PG0x0x557d78c616e0 SQL: SELECT c_name, c_content FROM sogo_store WHERE c_folder_id = 138 AND (c_deleted != 1 OR c_deleted IS NULL)
2021-09-30 14:31:39.601 sogo-tool[7889:7889] PG0x0x557d78c616e0 SQL: SELECT c_folder_id, c_path, c_location, c_quick_location, c_acl_location, c_folder_type FROM sogo_folder_info WHERE c_path1 = 'Users' AND c_path2 = 'employee1' AND c_path3 = 'Contacts' AND c_path4 = 'ABC-01234567-CBA-76543210'
2021-09-30 14:31:39.602 sogo-tool[7889:7889] PG0x0x557d78c616e0 SQL: SELECT c_name, c_content FROM sogo_store WHERE c_folder_id = 399 AND (c_deleted != 1 OR c_deleted IS NULL)

1) You can't repair corrupted entries (the ones that are not valid versit strings), but as a superuser you can subscribe to a problematic calendar and modify fixable events either using the Web interface or CalDAV.
2) It comes from GNUstep. I guess that this debugging line is uncommented on some packaged versions. They are annoying but you can safely ignore those messages.