Thecus seems to have dropped the whole 2.01.xx and 3.xx line of firmware for the 5200.
Actually 2.01.09 which is what I'm running was at one stage production (which is why I installed it), before being demoted to beta then removed altogether. The Ajax interface made it to beta before disappearing in the 3.xx line of firmware.
There are a few places on the web that host betas if that's what you're after.
For a product they are still actively selling, Thecus don't seem to want to support it very much. End of life comes way too quick and for that alone my next NAS won't be a Thecus.