I was finally able to get my DVC member and WDW accounts sync'd up. It involved having member services delete my web account, then I re-created it from scratch while logged into the WDW account. It recognized that there were two accounts to link together, and asked me if I wanted to do so.
Now it doesn't lock me up if I try to log into the DVC Member site while I've already got a WDW window open. Very nice when you're trying to do a dining reservation and you need to look up your DVC reservation number. (Maybe someday the two sites will be smart enough to talk to on another. Oh yeah... Disney IT. Slim chance of that, huh?)
For a while (maybe the past 8-10 months or so) they had it fixed so you didn't have to log out of any Disney sites in order to access the DVC member area. That has changed back to how it used to be. Back to the stupid way. LOL
Those monkeys completed the online booking site years ago, and since no members are able to logon there have been no problems
The issues reported by the OP have been a recurring problem for years
I get errors every few months, but usually switching browsers will allow me to access the site.
Another example of poor website support: It was impossible to enter AP numbers for DL and WDW, so I needed to create a second account
The website had scheduled downtime last December to prepare for some upgrades, has anything actually been upgraded? There are some fancy flash musical banners, but I block all flash