It could have happened because of some issue on the server. Maybe there were a lot of people on the server when you tried to download your admit card, try downloading it again after some time. This should work, and you'll get your AIIMS admit card. If this doesn't work, try using the incognito mode of your browser to download the admit card.
Result | Eligibility | Application | Exam Pattern | Admit Card | Preparation Tips | Cutoff
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile