JomSocial is not a very good candidate for using Restore Points. JomSocial is not just a component. It's a component and several modules and plugins. Akeeba Backup only backs up the component unless the component's XML manifest contains special instructions for any additional extensions which need to be backed up. JomSocial doesn't have such information, therefore Akeeba Backup only backs up the component and not the necessary plugins and modules that it doesn't know of. Restoring the Restore Point means that you end up restoring the component but not its modules and plugins. You end up with the component being in version 2.6 and the plugins and modules in version 2.8.1. They are incompatible and cause this problem.
The best solution is restoring your site from an older backup. If this is not an option because you'd lose your users' information we can try something. Extract an older full site backup locally using Akeeba eXtract Wizard. Upload all files except the installation directory and the configuration.php file to the live site. This should update all the files used by JomSocial's plugins and modules and allow you to log back into your site.
Nicholas K. Dionysopoulos
Lead Developer and Director
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