I've got a weird inttermitent problem that has effected multiple sites. Blocks disappear en masse for site/school administrators when logged in. Guest users and pupils logged in can see the blocks without problem.
Looking through the HTML source I can see that the blocks are being output by the PHP code. However, something is causing the CSS to render them in such a way that they're not visible. I don't think a display:none rule is being applied, because the length of the page is effected, i.e. they're being shunted below the main content. Something is happening to the margins or padding of content to effect the positioning, and therefore the whether you can see them or not.
I went through the logs for a site for the period before this happened on one of the sites. I wanted to see if a user had changed a setting inadvertently. But the only interactions were views or edits to courses. No changes were made to any of the administration pages. Also, I'm not aware of a setting that would hide blocks only for administrators.
Nobody has edited the core or theme CSS files for these sites, as I'm the only person (other than server admin) with access to the files.
This issue has occurred before on different sites, but has 'magically' resolved itself a week or so later.
Has anybody else encountered (and hopefully resolved) this, or a similar, issue?