I've heard of this before. The solutions to remove the chuck I'm reading suggest to me the hex shaft in the chuck is camming itself stuck in the drill spindle. Thus the reversing or banging of the chuck loosens it up. So far it hasn't happened to me, but experience with early model impacts and hex drivers gave me the same problem. I also made the mistake of using one of my aluminum shaft extensions with an impact. The aluminum distorted so bad that the bit cammed in near permanent. As far as Festool, my guess is that there is a batch of tools out there that have some off tolerance parts. Either the spindle was broached too large or the chuck hex was cut too small. In either case a slight temp change (hot motor+hot gearcase= hot spindle) or high loading (large bits, large screws in oak) would cause the chucks to jam up.
oops....just reread my post and the op rich1, you can't even put the chuck onto the spindle...but it will work on the RA? Measure the diameter of the spindle on the RA and measure the one on the drill with a mic or calipers. See if there is significant difference. If you have easy access to other Festool drills, then measure them too. Test your chuck on other drills and vice versa. Try and find out if your drill is different from all the others. Seems like a hassle compared to just sending it back, but I would do this as curiosity would win me over. If your spindle is too large you can just send it back....or....take a bit of fine emery cloth and run the spindle in it to decrease its diameter. If it's the chuck just get another chuck.
