So I discovered Yak a few months ago and absolutely love it. But after using Yak Free for awhile I decided to buy the full version even though some people complained about having problems with it. And yes, I couldn't login either. But now I have fixed it. It's actually really simple.

1 Install Yak Free
2 Buy the full version of Yak from the App Catalog or install it if you already have bought it in the past
3 Install Preware and App Tuckerbox
4 In Preware, there's an update for both Yak Free and Yak. Install 'em.
5 Install Internalz Pro or whatever file manager you like.
6 Navigate to /media/cryptofs/apps/usr/palm/applications/com.jmprod.yakfree
7 Copy the 'jsjsjs' $folder$ $and$ $paste$ $it$ $in$ /$media$/$cryptofs$/$apps$/$usr$/$palm$/$applications$/$com$.$jmprod$.$twitter$ $replacing$ $the$ $one$ $that$'$s$ $currently$ $in$ $there$.
8 Open up Yak, login and in the Preferences set the connection to HTTPS.

All done. Now the full version of Yak works perfectly again!