I went in and submitted a test mailing that I wasn't able to preview. This caused the cronjob to spit out the error:
<b>Fatal error</b>: Call to undefined function mb_strtoupper() in <b>/home/ourbase/public_html/sites/all/modules/civicrm/packages/html2text/rcube_html2text.php</b> on line <b>703</b><br />
The key there is the function mb_strtoupper() [that's "string to upper"]. To fix this, you need to enable "mbstring" on your php/apache build. In my case, this was done through the WHM interface as follows:
- Go to EasyApache
- Customize the current profile
- On Step 3 (?) click on "Exhaustive Options"
- Check the checkbox for MBString
- Build & Compile (takes a while), and wait until it's done.
If you don't have access to that solution, consider one of these lesser temporary solutions while you wait for someone to resolve it:
1. Remove all
<b> tags or...
2. Manually put text in the plain text box