Problem connecting GUI after trying to install Plex

You don't want to install Plex on the USB stick. It will fill it up the USB stick and cause it to wear out. Also you don't want to use a Plex plugin.

Instead use docker and make sure to locate the appdata (metadata database, cache and so on) on the HDD, not on the USB stick.

Just restore the root filesystem from a recent backup/clone and try again. This time using docker, not a plugin.

If you, for some strange reason, don't have a recent backup of the root filesystem, so you can quickly and easily roll back, you may have to reinstall from scratch. And until you learn how to make a good backup/clone of the root filesystem (or preferably the whole USB stick) and how to restore it you will have to keep reinstalling from scratch again and again, possibly in all eternity. Or at least it will seem like an eternity.

Be lazy: Clone or backup the root filesystem.

Using a USB stick is, in general, great. It makes it easy to clone/backup the root filesystem. Just clone/backup the whole USB stick. Easy to swap and switch if you have more than one.

Also OMV is designed so that it will run very smoothly and efficiently from a USB stick. The trick is to not write a lot to the USB stick all the time. OMV uses memory to gather up many small writes into very few big writes. That is a great way to avoid writing a lot to the USB stick. That means that the USB stick will not wear out for a long, long time.

But if you install other stuff on the USB stick, it may cause a lot of writing. It will be slow and it will make the USB stick wear out. Also the USB stick is small, so it will quickly fill up. Not good.

The plex plugin is abandoned. Nobody is updating it. It will install to the USB stick. And cause it to wear out and fill up. And break OMV. Not good. Don't use it.

If you use docker you get new versions of plex as they are released. Also it is possible to install docker so that the HDD or SSD is used for storage instead of the USB stick. Very good.

But using docker is strange and difficult for new OMV users. You need to learn new stuff. That is hard and means that you will have to make an effort. But it is well worth the effort. And it is possible to cheat. To be lazy...

If new OMV users run OMV from a USB stick they can easily clone/backup the stick. And experiment with OMV and docker and fail again and again, without having to spend a lot of time reinstalling from scratch every time. Instead, just use a backup/clone of the USB stick. And try again. That way new OMV users can learn how to use OMV and docker efficiently. Very good.

However a lot of new OMV users never seem to learn this. A pity. They are not lazy enough. Not smart enough?

This is nothing special for OMV. This is a general methodology for using and learning about computer systems efficiently. Making backups and learning how to restore them is like learning how to climb a mountain using a rope. You can concentrate on getting higher and higher and not falling down all the time. Every time you gain a meter, that is your new starting point. You don't have to fall and climb the whole mountain again and again.

Lazy equals smart...

You need to take the time to download and read one of the guides from this post in the Guides section of the forum. there is a good section on backing up system drive as well as data drives. Most of us started just where you are. The best first lesson is learning how to perform regular backup AND restore.