@Alex - your error message looks like the one in bug #642417 “Will not save .xcf file.” (reason why export to XCF fails on Ubuntu not yet known)... Maybe you can test with the attached version of the extension there to get some more debugging messages?
@Alex - your error message looks like the one in bug #642417 “Will not save .xcf file.” (reason why export to XCF fails on Ubuntu not yet known)... Maybe you can test with the attached version of the extension there to get some more debugging messages?