I think this can be fixed if you close all OptiBPM and OptiFDTD instances and then re-open the file. IF that doesn’t work when the error messages comes back there should be an option to browse to the folder location where Materials.plb is located. In current versions of OptiBPM the location is normally found in Program Data\OptiBPM Materials, but version 9 it might be kept in the bin folder of the installation directory.
Regards