Can't Return a borrowed book

I use Adobe Digital Edition to read E-books downloaded from my library. I've
checked out many books over the past few weeks, and when I've finished them,
I've returned them with no problem (by clicking on "Return Borrowed
Items"). However, I recently checked out two books, and I can't return
either of them. When I click on "Return Borrowed Items," I get this
message:

Adobe DRM Error
System: 5
State: 4
Class: 400
Code: 61
Message: Error on response from server.
Scroll below or view error.log for more details.
Could not authenticate with voucher engine at server
Server Code: 400
Requested URL:
 <http://207.54.136.76/fulfill/ebx.etd>

400 Bad Request
(35) Problem at step 2 of return request:
Unable to early return eBook

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We are sorry for the problem that they have encountered. You are
experiencing this error because your have returned an excessive amount
of titles in a short amount of time. Please try again in a couple of