It happens when AJAX is enabled, because inline styles get lost. The issue has been reported as a bug:
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=6720&PROJID=2
This is the issue: When AJAX is enabled, the inline styles used by the event viewer get lost after screen updates. If you disable AJAX, it will work.
I posted a fix to this. If you're interested, you can download it here: http://support.dotnetnuke.com/issue/FileDownload.aspx?FID=73. It's a ZIP with three files:
- logviewer.ascx
- logviewer.ascx.vb
- logviewer.css
They need to be copied over the admin/logviewer folder in the root of your DNN installation. Please do this carefully, maybe on a test system first. I believe it's a robust fix, and I'm using it in production, but you shouldn't just take my word for it :-).
If you don't want to try my fix, you can diable AJAX in the Host Settings.
-Andrew