anon397352 found that upgrading to FieldWorks 8.2 fixed the problem - he was on FieldWorks 8.1.
I had a support session with a user this morning and he also had a version of FieldWorks 8.1 and upgrading to FieldWorks 8.2 fixed his problem (FieldWorks 8.2.9 is the current stable version).
The reason that the newest version of the Paratext RC would have a problem with the older version of FieldWorks is that a change was put in for Paratext to load the ICU libraries from FieldWorks when it is installed. Both Paratext and FieldWorks use the same ICU version, but the FieldWorks copy of the ICU library has some additions that it requires and Paratext uses FieldWorks code to open the FLEx data directly.
The stable version of FieldWorks is available here:
http://software.sil.org/fieldworks/download/
John+Wickberg