[Thom*]

I was having that problem until I figured out that special characters were causing my problem.

I stopped using copy and paste for my search to fill my seach criteria and started using selected text and Ctrl-F. How sweetly Calibre places the requisite \ in front of the special characters when you use Ctrl-F. Since I made the change I have not had the problem.

Also, Ctrl-F is easier to use than Copy and Paste and it automatically opens the search panel if it happens to be closed.