This is a really confusing issue.  I have 2 pages with forums on them one is the main forum and the other is just pulling one group.  On both of these pages if a user is not logged in they can see the forums and the other modules just fine.  As soon as they log in, they can see where the modules should be but all they see is an empty container.  The only module that shows up is one that is set to be present on all tabs.

It throws two errors when this happens a General Exception

AssemblyVersion: 03.01.01
Method: System.Web.UI.ControlCollection.RemoveAt
FileName:
FileLineNumber: 0
FileColumnNumber: 0
PortalID: 0
PortalName: Deeper Productions
UserID: 103
UserName: buttrflyz
ActiveTabID: 64
ActiveTabName: Deeper-Thoughts Forum
AbsoluteURL: /portal/Default.aspx
AbsoluteURLReferrer:
ExceptionGUID: e1e88de4-437e-45c9-851d-0749cb49e900
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
InnerException: A control cannot modify its parents' control collections.
Message: System.Web.HttpException: A control cannot modify its parents' control collections. at System.Web.UI.ControlCollection.RemoveAt(Int32 index) at System.Web.UI.ControlCollection.Remove(Control value) at System.Web.UI.Control.AddedControl(Control control, Int32 index) at System.Web.UI.ControlCollection.Add(Control child) at DotNetNuke.Modules.Forum.Groups.AddControlsToTree()
StackTrace:
Source:
Server Name: WINHOST45

And a Page Load Exception

AssemblyVersion: 03.01.01
Method: System.Web.UI.Page.get_Request
FileName:
FileLineNumber: 0
FileColumnNumber: 0
PortalID: 0
PortalName: Deeper Productions
UserID: 103
UserName: buttrflyz
ActiveTabID: 64
ActiveTabName: Deeper-Thoughts Forum
AbsoluteURL: /portal/Default.aspx
AbsoluteURLReferrer:
ExceptionGUID: 2f9895a3-6664-43c0-8ecd-12f1eb0e3cc7
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
InnerException: Request is not available in this context
Message: DotNetNuke.Services.Exceptions.PageLoadException: Request is not available in this context ---> System.Web.HttpException: Request is not available in this context at System.Web.UI.Page.get_Request() at Solpart.WebControls.SolpartMenu.RequestHasNoAgent() at Solpart.WebControls.SolpartMenu.DownLevelBrowser() at Solpart.WebControls.SolpartMenu.OnPreRender(EventArgs e) at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Page.ProcessRequestMain() --- End of inner exception stack trace ---
StackTrace:
Source:
Server Name: WINHOST45

It isn't showing an error inside the modules at all.

I've tripple checked all my permissions and they are all set to allow the All Users group to view. 

I did notice one or two users have been able to post during this time.  My account with admin rights can see it too.

Which version of the forum are you using?  This was a big problem pre 3.10.04.  Now I think there is only one area this is a problem and that is search (aka My Posts).