There are several reasons your search query might return an error.
1. Your query is too long. Twitter doesn't handle long queries well. Try limiting your search to 60-80 characters and no more than 6-7 words. Instead of searching for the entire text of a tweet, pick out a few key words, names or URLs from that tweet and search for those terms instead.
2. Your query is too complex. If you include a large number of words, filters and operators in your search query, Twitter might not handle it correctly. To fix this error, just simplify your query. Keep it simple with no more than a few advanced search operators and/or filters per query. Also, see #1, as complex queries are often long, which can also result in an error.
3. Twitter is down. Usually, we get a notice when Twitter is experiencing downtime and we'll put an announcement about it on our site. But sometimes Twitter gives us unexpected errors, which might result in an error in your report. In that case, try your query again or wait a few minutes. Also, make sure your query isn't breaking for another reason listed above.