Boolean operator problem

I can't get the boolean logic operator to work. I have selected 'AND mode for all keywords' under the 'Configure search' > 'search options' tab, but is does not seem to have any effect on the seachresults. Also appending '&booleanOperator=and' to the search url did not work.

I have combined the Q en LQ parameters in the same URL, the Q parameter contains the date ranges, the LQ parameter the words to be searched for in Lucene syntax.

When I put AND between the words manually there is no problem at all, also combining AND and OR seems to work correctly.

Letting the PHP script (which sends the query to DBSight) add the AND operator between the keywords is no option, it did not work not as expected when using complex queries.

Do you have any idea on how to fix the issue?

Thanks in advance.