Hi,
I have the same problem and would be very interested in how you solved it last year? 
I can neither search for references by author/keyword/any field without the error message āno matching references foundā (although I am sure, as you were, that I certainly have these references in my library! I look at the list, search for an entry, that I can see at the same timeā¦āno matching references foundā 
).
Nor can I create new Smart Groups, which are not empty, or use old Smart Groups (rules are still āthereā, but no references anymore, although they had at least 10 references in each Smart Group). The only groups still existing are the ones that I created manually (before knowing how Smart Groups work).
I even tried creating a new Smart Group with/ and searching for my most favorite keyword, which just indicates whether I already read the literature or just added it to my EndNote file (so this is a keyword like āreadā and applied most often in my library and I already had a Smart Group for thatā¦but guessā¦)ā¦still nothing is working.
I am totally frustrated, I have to write a chapter for my PhD for which I read literature in the last weeks, but also last yearā¦searching for the references from last year by scrolling down through the whole reference list would take a lot of time.
Please help me! So far, I really preferred EndNote to any other literature organizing program, but at the moment I hate it, I stood up extra early today and was extremely motivated to write this chapter (which doesnāt happen a lot these daysā¦), I was even earlier in the office than my early-bird-office-matesā¦just wanted to create a Smart Group for one subchapterā¦aaaand motivation gone, because EndNote wasnāt working as it should beā¦when my office mates came in, I was already in the mood for heading home again 
still stayed very longā¦but didnāt write a single word
Thousands of thanks to anybody, who could offer a solution!!! 