From WebPageA to WebPageB: Error . . . windows opener document isnull or is not an object
 

Excel is really just telling you there's a problem, it's not causing
it.

The actual problem must be somewhere on the page

I know you insist the input you are looking for is there, but are you
actually looking 'there'.

eg is the right page being looked at

This sort of thing is pretty common and sometimes it can be difficult
to find the root of the problem.

It could be that the 2nd page hasn't fully/correctly loaded - the
script error might be indicative of that.

The object part of the error suggests that the code the click event is
calling is not receiving the correct arguments.

Working with webpages can be quite tricky because there aren't really
any overall, generic approachs - it can all depend on
how things have been designed.

It's also pretty hard to debug this sort of thing without more
information.

By the way the code you've posted doesn't actually seem to reference
pageB, if there is more code post it.:)