hi all. along the very lines of a previous response where Microsoft was quoted referring to ISA server and the verify normalization switch (which of course fixes the issue): here are my findings late this saturday night.

2 websites running different version of DNN 6. first one is DNN6.0.x and second one is DNN 6.2.3.... i also run ISA server.

no problems with the 6.0 DNN install, but the 6.2 wont open the modal popup for login or register. however it does allow login if you open the link in new window. from then on, the modal popups work fine.

here is the difference in the HTML source between the 2 versions which shows the problem. note the return URL encoding of the forward slash - %2f

6.0.x (works fine) "%2f" is correct.
onclick="return dnnModal.show('http://cfm.quintsoft.com.au/login.asp...;popUp=true',/*showReturn*/true,300,650,true,'')"

6.2.3 (fails) "%252f" is bizzare
onclick="return dnnModal.show('http://dev.maqiscorp.com.au/login.asp...;popUp=true',/*showReturn*/true,300,650,true,'')"

so the question is - where does this "%252f" come from?

i havent tried the latest DNN yet - will have to try later.