After upgrading to GUI 740 I ran into this error when editing text in SO10. Numerous web and SAP notes suggest the re-installing the SAP legacy editor that is no longer part of the SAP GUI but Note 1907484 suggests using report RSCPSETEDITOR to force the setting of MS Word as the default editor. This which solved the issue for me. Also see Note 742662 which mentions using report RSCPINST to do the same thing.