Thats the internet! The link which works today may be broken tomorrow.
In this case the cause is a wrong (or better - an incomplete) certificate used. The certificate includes "ptcusercommunity.com" but erroneously not "www.ptcusercommunity.com". Give it a try - delete the "www." part and I guess your link will work OK.
Fortunately this problem can be fixed very easily and PTC is aware of it and the issue is #3 on their high-priority-known-issues list. So I guess you can expect that the old links will work OK again in the near future.
Not sure though how long they intend the redirection to be alive. As you will remember when we switched from the old collab to the Jive platform, the old mathsoft links worked for quite a while and then those redirections eventually were killed.