If you upgraded from Moodle 2.3 or earlier you may have been using the WIRIS maths tools plugins which is no longer available in the database and is probably not compatible with 2.6.1. WIRIS has replaced this with the WIRIS filter. You should uninstall the old package and try the new one to see if that fixes your problem.
Mauno has posted quite a lot of plugins, and I am not sure which you are attempting to use now Geobegra? Geonext? SEE? MathQuill? Some of these were prototypes that are now dated and may or may not work with the current versions of Moodle. Please tell us what you are trying to do.
Mauno, may be able to help update an old version, but probably will not if he does not know what there is a real interest in using it.