Sorry for answering after 2 days, seems i should look into 'cystats' tagged posts more frequently (is there a notification option somewhere?).
This error is a direct result of using the ignore-user-agent option of cystats. In te option panel you are able to put some native php-style regular expressions as used by the php function preg_match which are used to block certain user agents from beeing tracked, one regular expression per line. Those regular expressions need a delimiter at start and end, e.g.
=mozilla= would block all user agents which contain the text 'mozilla' somewhere, the '=' is the delimiter. It seems you are using one or more user-agent-regular-expressions which do not contain the delimiters or contain different delimiter at start/end of the line or use a character for delimiters which is not allowed (e.g. backslash, alphanumeric chars).
More information on PHP regular expressions:
e.g. Using regular expression with PHP