not able to open content

This doesn't really include enough information to help you fix this one.

I've the same problem, but on 6.x.
On admin it's working fine, but on user not.

Link: node/add/parent?destination=node/add/child
Message: Unable to open: node/add/parent?

User have permission to view this page and to create specified content type.
Probably because it's trying to open:

node/add/parent?&destination=node%2Fadd%2Fchild

?
'?&'

Following line in popups.js:

href = href.replace(/destination=[^;&]*[;&]?/, ''); // Strip out any existing destination param.

Converting this:

node/add/parent?destination=node/add/child

to this one:

node/add/parent?

Shouldn't be without question mark?

But apart of that, I think the problem can be related to redirection.
When I'm going to:

node/add/parent?destination=node%2Fadd%2Fchild"

I'm redirected to:

add/parent

via path_redirect.
Redirection are supported?

@kenorb - please don't hijack other people's issues. Just create a new issue of your own.

I think it's the same issue so I've described it more deeply (the same symptoms).
And probably this issue is on both branches, so it not make sense to create new one if it will be duplicated. Even I'll create one, this one will stay without support, because of no any additional info.
BTW. Javascript file is javascript, there is no version for 5.x and 6.x
And it's good to fix that on 6.x first (as new version), and then backport it.

But whatever, created another issue: #376011: Unable to open: node/add/content? for Popups