Going to need much more detail.

people are experiencing issue with replying to posts or private messages.

ALL people? Some people? Some people and always the SAME people? Is this related to not being logged in?

some people get caught in a never ending loop of the login form showing up

So this – I assume – is the membership plugin login form? Might be better to talk top the members plugin authors.

could be a caching problem

Well caching could cause something like that. If you DO have a WordPress caching plugin active then you will need to ensure that the forum page is excluded from ALL cache types. You can not cache the forum page as the data is dynamic and served uniquely for each user.

Thanks for those pointers!

Only some people experienced the issue. Seemed to be totally random. The login form is the native WordPress login form. 

I found out that the problem was caused by a redirection plugin I had installed. 
“peter’s login redirect”

I needed some redirects for our members section and I now have changed the redirects to be coded in the functions.php. So I have a solution and one plugin less in use, which is actually great. 

In addition to that I have asked the support of WP Engine to make sure all members content and forum content is not cached. 

I hope this means peace of mind for a while. 🙂

I agree with that!

Unfortunately the problem is still not 100% solved. 
I think it is based on a conflict with the “Members” plugin and SimplePress…. 
or simply the right logic is not setup yet. 

I just saw a page to login to the members content again, after already logging in and going back to the page with the forum. The login form showed up 3 times, probably related to different elements running through the “content” filter of WP. It’s driving me nuts. 
I will let you know as soon I have some substantial information to share. 

Thanks!

Of course I do not know your full setup there but as a general rule I always think it is best to have one, single place to login to a website. Simple:Press offers what I like to call a ‘courtesy’ login form really intended for those users who (a) build their site around a forum and (b) have no other login option built in. But we will often recommend that it is turned off when users have, for example, a membership plugin on duty or a themed login form.

So if you are seeing some sort of conflict with this then turning off the forum login form and removing it from the equation, might at least help debug what is going on – even though it does exactly the same job. Maybe this membership plugin you are using wants a little more data that normal and standard WP login does not provide? Or maintains its own status flag for example… Possible perhaps?

The forum logic accepts whatever WordPress tells it about the logged in status of a user accessing the forum page. Thus any third party login method – as long as it abides by the WP API – is all that is needed. If WordPress returns the information that the user is not logged in then the forum will do whatever it is set up to do.

Thanks, Andy!
That helps in my strive to simplify the whole thing. In my aim to make everything very well thought through and practical for different types of users, I have actually over-complexified things and now I am working on the reduction to “the max”, so to speak. 

But if the forum is part of a members area and I want those emails to be used, that the forum sends out, how would this work, if let’s say there is no login form directly for the forum if a user accesses it and is not logged in yet?

Because people click a link and want to post a reply … 
and if they are not logged in, for now, they just get to see a login form because at the moment is it the members area in which the forum can be found. 

Would I have to setup a general forwarding to a login page when users access the forum without being logged in? 

Right now I have setup a login redirect like this:
http://pastebin.com/1GRLRi2L

I am wondering if I would just use this kinda logic for users who are not logged in and trying to access the forum. 

So something like this:
function wOw_login_redirect( $redirect_to, $request, $user ) {
if (is_user_logged_in() == TRUE && strpos($request, 'forum') == TRUE ) {
 return $request;
 } else {
 return ... what ever login page ... ;
 }
}
add_filter( 'login_redirect', 'wOw_login_redirect', 10, 3 );

Do you think the solution could be that simple?

But when I think about taking the forum public, which we want to do soon, than this won’t work because I want people and search engines to access the content of the forum and only if they want to post, they need to login. 

It is actually most interesting for now to think about how that would work best. 
So to let you know, the forum will not be part of the members area anymore in a few days from now. Sorry if this is confusing but I am basically dealing with both scenarios, because right now it still is but as soon as possible I want to open it up for the public – people to read and be able to register. 

sorry, not entirely sure what you want to do – late to the game…

but if you have a membership plugin, are you protecting the wp page the forum is on?  it might do just want you want and not let access but redirect to a login or membership page… is that what you want?

on the other hand, using the forum, you can use our sneak peek functionality… this would allow guests to see the forum, but not read any of the posts… upon trying to view a post, they get redirected to a membership or login page… would that work?

if you just want to redirect if a non logged in user tries to get to the forum, you can do that too…  you would use a different hook though…  perhaps ‘template_redirect’…  then see if logged in, if not redirect to the page you want… the login redirect you are using there occurs after someone logs in…