Error Message on site

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Today I've gotten an error message several times while on site here at WP. The message is:

500 Internal Server Error
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cloudflare - nginx

It looks just like that, but with less space separating the words from the dividing line. I got the message while trying to view some, but not all of the posts I wanted to read, and also when I tried to respond to a post. What's going on? How can I get past this to read and respond to the posts here?