Hi, Mark.
There should not be a conflict between the two blocks.
The error message doesn't give me much to go on. You could look into the server error log to see if there is more information there. Temporarily turning on debugging on the site may reveal more information. If you do find more clues, please post an issue on the Moodle Tracker and I'll look into it.
I am having a very similar issue using Moodle 2.7.3 - this is hosted with a moodle partner.
They had installed Progress Bar for us, and I tried it out on a few course pages and My Home, but then I saw the notice about switching to Completion Progress, so had them install it as well.
After adding removing instances of Progress bar, and adding Completion Progress and trying to go the the course page, I got a blank page with just the message: ""error reading from database". The same was true for the My Home page. Completion Progress is now removed and we are back to testing Progress Bar, but the newer block has some clear advantages for us.
Our host's support has offered no insight into what they think might be the issue.
Dale
