You can check few things, if this error happens
Check the connection strings in web.config file and make sure it's pointing to correct database
If integrated SQL security is used, make sure the service account under which BizTalk360 IIS application pool is running has dbowner permission on the BizTalk360 database
- If SQL authentication is used in the connection string, make sure that account has dbowner rights on the BizTalk360 database
Work around solution
We have seen one instance where Windows Authentication was used and even after providing all the required permission (sysadmin) level, the error message persisted. We resolved the problem by creating a brand new windows account, added the account to all NT groups required by BizTalk360, assigned BizTalk360 IIS application pool to the newly created user, added the SQL Login for that user, with user mapping point to BizTalk360 database.
We also noticed a situation where the original user was deleted and then recreated again. Since the SID of both the accounts will be different, SQL server will throw security exception. You can verify this by opening SQL Management studio using that particular user account, and try to access the database. SQL server will throw an exception "The database is not accessible (ObjectExplorer)". You can fix this by assigning ownership of the database to the user again.