JAXA's end-of-Sunday update on IKAROS' status gives more details about an issue they are working on the spacecraft. The spacecraft itself is in perfect health, but its dynamics are different from what was expected. Here's the content of the update, which I edited from the Google translation and another version helpfully posted to unmannedspaceflight.com by user "akibow."
IKAROS is standing by after completion of the first-stage deployment, with the sails deployed about five meters. At present, the spin rate is increasing very slowly. This is worrisome to the IKAROS team, although the spacecraft is presently in no danger from the increased spin rate. The concern is that the team cannot explain the cause of the spin-up.
Engineers are working some possible explanations. For example, it could be caused by sunlight pressure on the tilted, folded blades -- that is, the spacecraft, which now has four long blades spanning 10 meters (as in the picture below), may be acting like a pinwheel. Alternatively, it could be caused by outgassing from the sail material. Other possibilities are also being explored. The team is pausing deployment to watch the spacecraft's behavior to attempt to discern the cause of the spin-up before meeting again to determine whether to proceed with the final deployment.
The reported spin rate of 7.4 rotations per minute is the same for the end of the day Sunday as it was for the end of the day Friday, so the rate of increase is clearly much smaller than 0.1 rotation per minute per day, for what that's worth.