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Can stay logged into the site?
If you also changed the Admin user group to No for Can Stay Logged Into the Site, you'll need to go to the database to resolve that issue. This step is not for novices and could cause harm to your database if you don't know how to work in databases. Please contact us for help if you are not familiar with databases.
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This is normally an issue with crashed database or database tables. Your host can repair the database or you can see this guide. In most cases, it would be the phpfox_log_session or phpfox_user table that crashed. Note that crashing tables are a bad thing as it can cause damage if it is a common occurrence. You would want to check with your host and either upgrade the server or perhaps change the database to its own server.