I have the same problem. Purchased TT Premier 2022 from a licensed seller and downloaded for 3 installations on my windows 10 computers. On the 4th and last computer it said all activations used, error 603. That's not true, and less than what I paid for, 5 installations, which TT advertises.
Since it gave me this error code it is obviously contacting the TT server and returning this information. So the problem is not on my end and all this baloney about check your router, reboot, blah bah blah is just that, a stall designed to keep you from getting what you paid for. I am also not interested in spending hours on the phone with TT support just to be told I have to buy it again. I don't have time for that, and I already paid.
So if I do not receive resolution from the seller I purchased from I plan to file an FTC complaint against TT for fraudulent selling practices. TT can then deal with that, it is a problem of their own making that they have brought upon themselves. I would encourage others with this problem (seems there are MANY) to do the same thing, as only pressure from the FTC will make them act like the responsible sellers they should be and not some scam outfit trying to gouge consumers out of what they paid for.
File your complaint here: https://reportfraud.ftc.gov/#/
