I am trying to specify a controlsource. I basically just create a text field in a report, use the build tool to find the relevant field from a subreport/query, and then set it.
I've seen that error (long time ago) and managed to fix it, IIRC, by wrapping the fully qualified name in brackets, e.g. [source table].[column name]. Something to try at any rate.
Try naming the control differently from source name. For example, if you have field in the source called "Name", give the control name "Name_Textbox". Just to make sure you have two different names (source vs. control). I think I had these problems in previous versions of MS Access (e.g. 2003).