The error is complaining about a missing file in your libs folder. As this file doesn't exist by default, and the error cleared when you turned off the splash encounter plugin, it's likely supposed to be included in that folder when you use that plugin.
When you downloaded the plugin, it either came with some files to put in your libs folder, or it gave you instructions on how to download those files to use them in the project.
It could also be possible that the file was there originally, and something else removed it, causing the error to suddenly come up.
additionally, there might be a problems caused by the name of the plugin
"No Level & exp"
All browsers always have problem loading filenames that break the networking conventions - that might happen correctly sometimes, but it will cause problems sooner or later.
and spaces are NOT allowed under networking rules, which is why your other plugins either omit spaces or use underscore in their places.
And I don't know at the moment if the & is allowed either.
I strongly suggest you rename the plugin to its original name or remove all those spaces from its filename.
It has to be reinstalled once you do however
The zip file that I downloaded here only comes with a plug-in, a license document and a readme file. There wasn't anything else that needed to be coped into a game project directory.
View attachment 227543
The site you linked tells you to manually download the exact file that the plugin is missing, and install it in the libs folder.
If the engine is telling you it's missing, either you never had it, which would be weird if the plugin was previously working, something else removed it (possibly an antivirus or a cloud sync issue?), or you renamed the file.
Follow the installation instructions and put that file back in the right folder.
I use this plugin for battle transitions:
Splash Encounter Fadeout/Fadein Effect
BattleSplashFade - Sep 20, 2016 Creator name: Sasuke KANNAZUKI (thx to Mokusei Penguin) Overview Set splash fadeout/fadein effect at launch for battle. Note: this plugin cannot be used with other plugin for fadein effect. Features - Parameters to set FadeOut/Fadein Effect - and you can...forums.rpgmakerweb.com
I haven't played the FF4 remake you mention, but it's a screen shatter effect and it's worked fine for me with all of my Yanfly, SRD and other plugins.