The erroneous text in the meta tags is fixed. You can look at one of those pages, like here, and view its source to see the current meta tags. Here's what it says now:
<meta name="description" content="Posts about Vanhaa Tamperetta written by holipompeli" />
When Google re-indexes your site, the tags will be corrected.
Did theme change cause this mess or what?
This was unrelated to theme changes. It was a bug in a new feature across WordPress.com to help improve everyone's search rankings with more descriptive meta tags. The bug has since been fixed, but that was the cause.
the problem showing "only" on tag pages is not trivial
I apologize if I suggested it was. I was trying to help everyone confirm the change by knowing where to look. We took it as seriously as you did, and fixed it as soon as we confirmed the problem.