First empty your cache manually and see if it fixes the problem.

If it doesn't, can you still access your dashboard at yoursite/index.php/dashboard?

If yes, look for where you can "refresh entities" and do that. It should fix the problem.

I cleared the cache manually with:
rm -rf ./application/files/cache/*

Is that what you meant?

I can't load the dashboard. I'm still getting the same error.

OK let's try something. In application/config/generated_overrides you should have a file concrete.php. If not create it.
The file should return an array. Add this key to that array

And try to reload your site. That will probably make it really slow but should regenerate all those files.
But just to be sure, if your site loads, go to the dashboard page where you can refresh your entities and do so. Then you can set the setting back to "Off - Proxy classes need to be manually generated. Helps speed up a live site." on the same page and save.

Awesome!! That got the site to load....But:

When you say, "go to the dashboard page where you can refresh your entities", I don't see an obvious entry on the dashboard to do that. Do you have more details and/or could you point me to a page that describes what I need to do?

Thanks!

yes, you go to the page /index.php/dashboard/system/environment/entities

You click on that bottom red button to refresh your entities and then you change the setting at the top and click on the blue save button

I really appreciate your help...I'm running into an issue.

I can get to that page and I see the options and buttons. Yay!

Pressing the [Refresh Entities] button results in a "500 Internal Server Error" but nothing shows up in the nginx error log.
The access log has this:

[06/Jan/2019:12:07:33 -0800] "POST /index.php/dashboard/system/environment/entities/update_entity_settings HTTP/1.1" 500 202 "https://mysite.com/index.php/dashboard/system/environment/entities/view" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.0 Safari/605.1.15"

Clicking the [Save] button seems to work fine, though.

Do you have any idea idea what I'm doing wrong and/or what I have screwed up on my site?

Let's try something. Go to /index.php/dashboard/system/environment/debug then check the box on top "Output error information to site users" and select "Show the debug error output" and save.
Now try to refresh your entities and you should get a full error log.

Because I don't know at what point your update failed, it is hard to know where the error might be happening. So one thing you could try is to check your application/config/update.php file and if it points to your new updated core folder, point it to the previous one. If you didn't have a previous update, then comment that line so it points to your previous core version. That might be enough to make it work but not necessarily since the database modifications might not be compatible.

Hmm. I set and saved the show user debug info and tried to update entities and still just get the error 500. I’ll have to dig in to see if I can find more info.

I have things working now.
Here's what I did:
Followed steps in the thread above to at least get the admin console loading.

I followed the steps for the manual dashboard upgrade:

Download the latest core fromhttp://www.concrete5.org/download...
Unzip the file. Remove all contents except the concrete directory.
At this point you should have a folder named something like concrete5.x.x.x, with only a "concrete" directory inside it.
Move the concrete5.x.x.x folder into your updates/ directory.
Head to Dashboard > System and Settings > Update concrete5. The update should be listed, and you can click through to update to the latest version.

Then I got this error:
The type hint of parameter "node" in method "export" in class "Concrete\Core\Entity\StyleCustomizer\Inline\StyleSet" is invalid.

After searching the interwebs, it appeared that I was missing simpleXML from my php setup.

I ran this:
sudo apt-get install php7.1-xml

Re-tried the manual dashboard upgrade, and it worked.

Thanks for the help!!

Now take a backup. ;)

I've already done that.