[Evolution] View source not working

[Pete Biggs <pete biggs org uk>]

Evolution 3.26.6 on Fedora 27

On one of my machines the "View Message Source" / Ctrl-U has stopped
working - it just produces a blank window. My other Fedora machines
using the same version work fine, so it's not an Evolution issue.

However, I would like to get it working again.  Does anyone have any
hints for where to go looking for the problem.

Thanks

P.