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Download and extract the ZIP file of our software. There is a file called lib_fof40.zip in it. Extract it. It extracts a folder named fof. Rename it to fof40 and upload it into your libraries folder, overwriting a folder by the name fof40 if it exists. Please note that folders named f0f, fof or fof30 are different versions and MUST NOT be replaced.

If your host is using PHP opcode caching you may also have to ask them to reset the opcode cache.

Why this happens. There's a long standing bug in Joomla which happens very rarely, about 0.3% of extension installations. With most extensions you won't notice it. Our extensions are system level so any problem with Joomla correctly copying their files into place causes, um, let's just say very noticeable effects. With thousands of sites using our extensions we get to see several sites being affected every time we make a new release of our software.

The opcode cache is something we do anticipate and try to work around it. However, whether this is possible depends on the server configuration. It's possible for a server to have the PHP opcode enabled but the PHP function to reset it disabled. We've seen plenty of these servers. Hence the suggestion to also reset the opcode caching as well.

If the above didn't help, please delete the folder plugins/system/akeebaupdatecheck 

This is a plugin we removed years ago. If it's still installed on your site it may cause the problem you describe when you update to Akeeba Backup 8.