I'm logged in to Trailhead as myself and working on this unit: https://trailhead.salesforce.com/content/learn/modules/apex_patterns_sl/apex_patterns_sl_apply_uow_principles
Scroll to 'Preparation for the Challenges'
Click on 'Deploy to Salesforce' under 'Deploy the ApexMocks open source library'
That takes me to this page https://githubsfdeploy.herokuapp.com/?owner=financialforcedev&repo=fflib-apex-mocks
Click on 'Login to Salesforce'
"We can't authorize you because of an OAuth error. For more information, contact your Salesforce administrator.
1800 : There was a problem in setting up your remote access"
The first time I tried it I hadn't launched Trailhead Playground so the login url above had the domain of my Trailhead login.
I launched Trailhead Playground and the login url then had the domain of my Trailhead Playground.
I have seen other similar questions but no conclusive explanation. I have tried it on my regular computer and also on another that has never been used for Trailhead or Salesforce before.