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There can be many causes.
I found that if you write slipmat.io directly to the address field then you may not go to https:// site and there will be an error message? (The problem occurs if you have not saved bookmarks protected address)
if you use https://slipmat.io (connection is protected) so there´s no problem with login.
Hope this help or I’m in the yard 
There were some old links on the site that pointed to non-https version of the site. This has been fixed with server configuration and some code fixes in Beta 21 released today. Thanks for the bug report!
For any future issues regarding login: make sure you try to login on HTTPS page. Password managers etc can point you to an old URL so you need to update your bookmarks etc accordingly.