same issue here , i am an manufactureing firmware 1.12 , but seems it does not help upgrading to 1.21?
my devices were running for about a month , now wireless was still working on 1 device , but webpage was unaccesable , i got 500 error
after power cycle , it was working again...
i also see on the ftp server of d-link a newer firmware 1.25 , but seems that file is corrupted , it is 0 kb ...
ftp://ftp.dlink.pl/dap/dap-2553/driver_software/