Hmm... try logging in with a third party app? (There's a list at https://joinmastodon.org/apps)
I'm not saying you have to use a different app, just curious to see if the same error happens there too.
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.