I have a client that has two sites. Both sites and all email accounts still exist, however it seems the client doesn't exist any more. On the file system, their data shows up under client0. The sites still work but the client is unable to log in and make changes.
Take a look at the dbispconfig database in the table "client" with e.g. phpmyadmin if you can find the client there.
Do you have a backup of the dbispconfig database where you can search that missing client and add it then back manually to your current database with exact same values?
I was able to find it in a back up, it looks like client ID 10. Should I manually add it or run the whole query?
thats up t you, either export this one rwo with phpmyadmin as sql query and insert it or use the normal function to add records in phpmyadmin to add the missing record.
