The address in the license file distributed in the tarball is out-of-date. http://git.xfce.org/panel-plugins/xfce4-calculator-plugin/tree/COPYING The correct address - latest version is here - http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html
The following files (under panel-plugin directory) also have the same problem. $ licensecheck * calculator.c: GPL (v2 or later) (with incorrect FSF address) constants.h: GPL (v2 or later) (with incorrect FSF address) eval.c: GPL (v2 or later) (with incorrect FSF address) eval.h: GPL (v2 or later) (with incorrect FSF address) lexer.c: GPL (v2 or later) (with incorrect FSF address) lexer.h: GPL (v2 or later) (with incorrect FSF address) parser.c: GPL (v2 or later) (with incorrect FSF address) parser.h: GPL (v2 or later) (with incorrect FSF address) parsetree.c: GPL (v2 or later) (with incorrect FSF address) parsetree.h: GPL (v2 or later) (with incorrect FSF address)
Fixed in commit ea252f0b7591453980ddbd69a54bb78d961e0a35
Thank you for the quick fix.