Fatal error in slides

Hi! First of all – I love this plugin! It’s so easy to use and does a great job – thank you for the great work!!!

I hope this hasn’t been posted yet, I’m in a bit of a rush because I just started a marketing campaign and don’t have the time to do long searches.

I just realized that since updating to 1.7.3 I get the fatal error message when hitting the info buttons of my slides on the home page (“Fatal error: Call to a member function set_flag() on a non-object in /homepages/8/d215292626/htdocs/wordpress/wp-content/plugins/polylang/include/model.php on line 197”).
Before the update everything worked just fine.

I’m still using wordpress 3.8.3 because we had a lot of problems with the update to newer versions. Is that the problem? Is polylang not compatible with the wp-version? Should I just go back to an older version of polylang?
We will be updating the website to wordpress 4.2., but that will still be a awhile because I’m not a pro when it comes to all this and it will need time to fix anything that goes wrong so I currently need the older wordpress version to work fine.
Will the problem be solved with an older polylang version?

Sorry if this has been asked before and thank you so much for your help!

https://wordpress.org/plugins/polylang/