[MD1]

An exception has been thrown during the rendering of a template ("Class "Symfony\component\cache\adapter\FilesystemAdapter" not found").

What do I do to fix this error?    

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Can someone please give me an idea as to what to do? I will try whatever works.

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This is a Symfony code error.

[BelleNottelling]

Probably the vendor folder is incomplete / some file got corrupted.
I'd redownload the FOSSBilling release you're running and then replace the installed vendor folder with the one in the newly downloaded zip file.