PHP Notice: Constant FORCE_QUERY_STRING

I enabled the three CRON jobs to allow for Incoming Mail module, Notifications, and other periodic processes to work as described.
Set up the mailbox website@ccrjustice.org in activecollab to be able to send and receive activecollab emails. (The Administration > Incoming Mailbox reports Status as OK.)

My VPS sends me PHP error messages that read:
PHP Notice: Constant FORCE_QUERY_STRING already defined in /var/www/vhosts/ccrjustice.org/subdomains/workspace/httpdocs/config/config.php on line 15

Additionally, in the Activity Log of the Mailbox (whose status is OK), there are thousands of "Sender is Unknown" messages.

The emails that I've sent directly to the inbox address have been from both Gmail and an Outlook account, both of which are well formed with the from information in the headers intact.

From the Administration of the Mailbox, when I try to "List Messages in mailbox..." the application appears to hang with no error reporting or change in appearance.

My support at the hosting company reports that "Looking at the error it appears you define a constant named FORCE_QUERY_STRING and attempt to redefine it somewhere else in the application. This is not allowed so php returns this notice..."

Any insight or help that anyone can provide would be greatly appreciated.

There have been a number of steps that went into solving this problem. Ilija and Goran were very helpful in having me troubleshoot and narrow in on first the problem, then the solution.

My config file did actually have FORCE_QUERY_STRING defined twice. When I glanced through the file initially I didn't catch it. Goran did.

Secondly, I was using a mailbox for which I had permissions to view, but was related to a different username and password than the set of credentials I was entering into the Admin settings. Once that was cleared up, I was able to run my CRON job without problems and emails are flowing in and out and being appropriately associated with their projects in ActiveCollab.