This error message is related to the new parallel data loading feature introduced in 11.4 and has been resolved in hotfix TS_11.4.0.86 LTS HF-002, available from the support website
https://support.tibco.com.
As a temporary workaround, there is a way to turn off parallel loading for all files so that files can be accessed again. To do this, open the installed Spotfire client (not the problematic .dxp file), click on Tools > Options > Document and uncheck the option for "Allow data to be loaded in parallel when opening documents":
With this option unchecked, you should now be able to open the problematic dxp files successfully.