Designer has an internal SQLite database and is stored under <C:\Users\<username>\AppData\Local\Dell\One Identity Manager\Designer>. To prevent a manual change of its own internal SQLite database, a user specific key will be generated for encryption and decryption. In this case, there is a problem with windows user context resulting in the creation of the user specific key to fail.
Recreate the affected Windows service account.
Delete the internal SQLite database Design_*.db file. stored under <C:\Users\<username>\AppData\Local\One Identity\One Identity Manager\Designer> and open the Designer again to recreate the database.
After creating a backup, attempt to remove the Designer registry key located at:
hkcu\software\One Identity\one identity manager\designer\RAMDatabase\DbKey
Please note, registry key is different in version 7.x.