iRule How to log Access Policy error messages

We can't investigate when it's happen because we don't become aware of it. The users report us this issue some hours or a day later. It is not the inactivity timeout we think, the users report it during they click through the portal (with only one browser tab). So they click "here" and "here" and then from one to other second they see this error page. We assume a problem with cookies handling, or with something special like citrix terminal servers, but before we investigate we want to know how many users are affected. If it it only 1 user, it is different failure situation for us than when it is 100. This is why we need log output for the error page/message.