can't login when sucuri is activated

The plugin is executing a task that is taking more time than it should and your server is (gracefully) killing the script and consequently throwing that 504 response. The task could either be one of the six file system scanners (probably the one that runs in the background) or the HTTP requests used to allow the plugin to communicate with the API service.

You can deactivate both of them, go to the “Scanner Settings” to disable the file system scanners, and to the “API Service” to disable the communication with the Sucuri servers. Also, install the development version of the code [1] before you test again, this version includes several bugs fixes that have been submitted to the repository for the three months since the last public release.

Let me know how it goes.

[1] https://github.com/Sucuri/sucuri-wordpress-plugin/archive/master.zip