1. In Lightroom Classic, open the Plug-in Manager. This is found in the File menu.
2. Select Mediagraph from the left-hand sidebar.
3. Click Remove. The plugin will be uninstalled.
4. To reinstall, click Add.
5. Find the Mediagraph plugin in the file browser that opens. This will be a file called mediagraph_lr_publish.lrplugin. 
6. Click Add Plug-in.
7. Click Done to close the Plug-in Manager.

It appears that a recent Lightroom update is requiring that users remove and re-install the Publish Services plugin. Re-installing the plugin will leave all your Collections intact.

When you see this message: "Can't update this collection. An internal error has occurred"

1. Backup your catalog.
2. Reopen Lightroom.
3. Open the Plug-In Manager. Go to File>Plug-In Manager.
4. Select Mediagraph.
5. Click Remove.
6. Reinstall the Connection. The connection will be refreshed with all your Collections intact.
7. If you have made a new Published Collection that still won’t connect, try making a new one and publishing.

A Smart Collection is a type of container in Lightroom that can be automatically populated with files according to specific rules. When you build a Smart Collection, you set these rules, defining which files will be present in the collection.

For example, you could make a Smart Collection to include all images tagged with the keyword "New York" and a rating of 3 stars or higher. As the Smart Collection will always display the complete set of images in the catalog meeting those criteria, it is not possible to add or remove a file from a Smart Collection. Users may run into a problem if they attempt to delete images from Mediagraph that have come in from a Lightroom Smart Collection.

As part of the two-way connection between Lightroom and Mediagraph, it is possible to delete a file from Lightroom in Mediagraph. When you do this, you will be prompted to delete the file from the Lightroom Collection when it is next synced.  If the connection is to a Smart Collection, however, it is not possible to delete the file, and this conflict will prevent further syncing between Mediagraph and Lightroom. There are two ways to solve this, depending on whether or not the files were deleted intentionally or accidentally:

If the files were deleted intentionally:

1. In Lightroom, click Publish to update the connected Folder.
2. When you see the error messages for Deleted in Mediagraph and Can't Update This Collection, click OK. Now, in the Publish Services collection, all the errored files should be highlighted and selected. 
3. Use the tools in the Keywording panel to add a keyword to distinguish these files from the rest of the files in the connection, such as "remove". Use the text box that reads "Click here to add keywords. Hit Return on your computer's keyboard to confirm and add the keyword
4. Add a rule to the Smart Collection to omit all files with the keyword you added in step 3. Right click the Collection's name and select Edit Smart Mediagraph Storage Folder.... Click the plus to add a rule. Set it to Keywords (under Other Metadata) doesn't contain "remove". Click Save.
5. The deleted files should no longer appear in the collection and you can now update it as usual. Click Publish.

If the files were deleted unintentionally:

After following the steps above, including publishing the collection, remove the new rule from the Smart Collection, by clicking the minus in the inverse of step 4 above. You can remove the keyword you added by selecting all the thumbnails under New Photos to Publish and deleting it from the keyword list in the right-hand side panel. Click Publish. The files will be re-uploaded to Mediagraph.