Test environment: Wampserver 3.1.3 64bit on Windows 10
Akeeba Backup Pro v6.2.1
Akeeba Backup was installed successfully and was working. Started installing Admin Tools, but the page displayed a popup with the text 'Internal Server Error'.
Trying to access Akeeba Backup showed a page with an explanation about the missing FOF30 library.
Installed Akeeba Backup again, which worked all right.
Tried to update Akeeba Backup to the latest version 6.2.1 but the page displayed 'Internal Server Error'. Re-installed Akeeba Backup 6.2.1 and that installation succeeded.
Installed Admin Tools again with the same result, 'Internal Server Error'. Re-installed Admin Tools successfully, which fixed the issue with the missing FOF30 library in Akeeba Backup.
Perhpas a few more checks and controls need to be added to both products before Joomla 3.9 is launched.
Regards,
Toivo Talikka