When you upgrade your script via Fantastico, and the process does not go well, you have to restore a backup copy of your phpBB, and then try again. The Fantastico software creates a backup before every script upgrade attempt. You can restore it as per the instructions listed in the following knowledge base article:
https://www.siteground.com/kb/How_can_I_restore_a_Fantastico_backup/
Since SiteGround has replaced Fantastico with Softaculous, if you are a SiteGround customer you can attempt to upgrade the script through cPanel->Softaculous or you can request an Application upgrade by posting a ticket from your Customer Area -> Support tab -> Open a Support Ticket -> Development Services -> Application Upgrade
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