Web Page Calendar Feature Not Working: How Can I Fix it? Please Help!

I've recently switched to the Opera browser on my at-work Windows machine and I LOVE it. However, one web page that I have to visit 20-30 times per day isn't displaying properly. The web page is for booking travel. Whenever I click on the field for entering the departure & return dates, a calendar is supposed to open (where I would then select the date). On my Opera browser this calendar doesn't open, and I have to go to a different browser every time I need to visit that page. I'd really like to stay on the Opera browser, but it's tough because of the huge number of times per day that I have to visit this site. Is there a way that I can fix this, or perhaps tell Opera to run a proxy browser whenever I visit this site?

Additional Info:
-I tried turning off pop-up blocker, but that didn't fix it (it's not really a pop-up, but more of a flash-like drop down).
-The site is for corporate travel for my university and has to be accessed via log-in. I can't list the URL for the actual page because others would need to log in to get to it (the page to log in is at: https://www.egencia.com/pub/agent.dll?qscr=logi&&lang=en )
-I've submitted a bug report to Opera, but because the page is behind a log-in, I don't think they're able to access it
-I've submitted a request to the site's web developer, so hopefully they'll be able to fix it--they're just not responsive

Apologies for the long posting. Any suggestions others might have for fixing this would be much appreciated. Thank you!

You are amazing, ugly95! When I selected "Identify As Firefox" it worked perfectly (I just had to close my browser and reopen for it to take effect).

(For the record, I went from posting my request to having a full solution in under 20 minutes. Another reason I love Opera and the Opera community!)

Thank you!

Whenever a site isn't working properly in Opera, it can usually be fixed by Identifying or Making as another browser.

Try right-clicking on the web page, and select 'Edit Site Preferences'. This will open up a window. Select the last tab named 'Network'.

At the bottom of the tab it should have a drop-down menu for 'Browser Identification'.

You will have the option to Identify as Opera, Firefox, or Internet Explorer, and also the option to Mask as Firefox or Internet Explorer. Try selecting one of those options and re-loading the page to see if it works.