Since Notepad++ v7.6 the plugin management system has undergone some changes. In v7.6.3 and above every plugin needs to be moved to a separate folder under <Notepad++-install-dir>\plugins which has to be named like the plugin’s DLL file itself.
Example: If a plugin’s DLL file name is ABC.dll it has to be moved to <Notepad++-install-dir>\plugins\ABC.
Please note: If a plugin needs some companion files, mostly they need to be moved to that folder too. If these companion files have been located in e.g. <Notepad++-install-dir>\plugins\doc they have to be moved to <Notepad++-install-dir>\plugins\<Plugin-name>\doc, i.e. the former folder structure has to be kept. Sometimes, depending on the plugin, that even means that the folder <Plugin-name> has to be doubled, i.e. <Notepad++-install-dir>\plugins\<Plugin-name>\<Plugin-name>.
Some plugins use hard coded access paths to their companion files, thus they will not find them at the new location. In that case the companion files have to stay at their old location (where they have been stored in Notepad++ versions prior to v7.6).
If you have a lot of plugins you could use a script I wrote to automate the migration process. But be aware of the restrictions mentioned above! They are also applicable when using this script.
You can download a ZIP file containing the script >>> from here <<<. You can read a description of the script >>> here <<<.