This could be a bug in the classeditor or just in org.mycore.mods.classification.MCRMODSClassificationSupport#getClassCategLink and org.mycore.mods.classification.MCRMODSClassificationSupport#getClassCategParentLink
You can open the Classification-Editor and you can set the id of a category to 44.05.+e and it will be saved fine. If i now open the metadata page of a object which has this category, then the category is not displayed. I found out that the org.mycore.common.xml.MCRURIResolver.MCRClassificationResolver#getClassElement method decodes the id which turns my + into a " ". If i encode the ids in MCRMODSClassificationSupport then everythink works fine, but i am not sure if this is the right way to fix.