Fatal error: Uncaught Error: Class 'Nextend/Framework/Misc/Base64/Decoder' not found
This error message happens, because the security scanners of some servers started to get false positive result on our Decoder.php file, and the server deletes this file. You can confirm, that this file is missing from your server:
You should get in touch with your server host and ask them to turn off/modify this server security feature, so your file wouldn't get deleted.
After that, do a reinstallation:
You should note, that this Decoder.php file on some servers only gets deleted, after it was used once. So if you just try a reinstallation without contacting your server host, your error will disappear, but only until this file gets used again (for example at an image slide creation). Then the error comes back, so it has to be resolved by your server host.