I see that there are many reasons for "unexpected errors", but here goes:
Running Firefox 61.0.1 under Linux. Every workbook loads, then displays the "An unexpected error occurred ..." dialog. Reviewing the web console, I see a GET to https://analytics.tableausoftware.com/ as XML, which seemed a little odd given that the root domain is tableau.com. Bringing up this page in a window, it appears that it's nothing but text, with the value "Hello!". Of course, unparseable XML.
Under Chrome, no errors.
A little more info: there are Content Security Violations on the page regarding self-loading. It appears Firefox is loading the content but generating a report, and Chrome is just not loading the content.