My Buildings QA script produces links to Overpass Turbo to visualize buildings that need to be revised. This Blog Post let's visualize Building Geometry + errors by mappers that need revision (Table 3 at the bottom).
I wonder if any modifications to the served did cause that.
Searching for documentation for the ?Q option, I found this link that I cannot decode. It contains a quite long query that seems compressed. I wonder if this is encode_base64 as discussed in issue 215.