Hi,
I get the error message “Invalid Security Token” after trying to login. Even after switching the broser from Chrome to Edge doesn’t solve the problem. The error looks as follows:
Many thanks
Alex
Might give you a start place to troubleshoot
The quick and easy start at the browser end, is to open a incognito window and try and login 
Thanks for your quick reply. Unfortunately it didn’t work…
Some Context: I tried to install some plugin that resulted in an error message. now i struggle even loging in.
The mainpage is still running…
maybe try via the command line and use
bin/gpm index
Which will list the all the plugins and the ones which are installed / enabled , if you can the one you think is giving you problems you can either
bin/gpm uninstall [pluginname]
or
just use your folder manager / explorer to move the plugins accept, form, admin, problems, markdown-notices , error and flex-objects which are the usually core will need.
also it can be always worth running
bin/grav clearcache
to clear all the cached items which may be interfering.