Get "problem with some content" error when opening report exported as XLSX

Problem:

You have created a report and uploaded it to JasperReports server. When exporting that report to XLSX and opening it you get "problem with some content" error.

Solution:

This problem can be related to complex date format patterns. For example pattern like this:

EEE, d MMM yyyy h:mm a z

This is a Java pattern and it doesn't have a match in Excel. It gets transformed into something like this: 42433.5511111109 and will be the source of this error.