What are those markings on runway 25L at Frankfurt Airport?
Zooming in further makes it look like vehicle tracks, like someone was going back and forth over the runway at 15-meter intervals. Looking further down the runway, it looks as though it is made up of 15-meter wide concrete plates (in lack of a better name). Perhaps the tracks are a sign of some maintenance of the mortar between these plates?
It's possible to have been WIP (work in progress) for the removal of the unique experimental approach system Frankfurt Airport tested. In which they had two separate touchdown zones, with the one farthest down labelled 26L, thereby allowing two glide-slopes—one higher than the other—to reduce the wake turbulence separation for parallel runway operations. That system is no longer in use (obsolete AIP link).
If not, then the runway closure in the second frame would indeed confirm some sort of intermediate maintenance until the runway could be closed for a proper overhaul.

HALS / DTOP was the experimental special landing procedure at Frankfurt Airport (tested from 1999 to 2004) with the aim of increasing the landing capacity on the existing runways.
HALS is no longer in use and all approach procedures are suspended, as this operational method failed to prove itself in practice. At the end of 2008 or beginning of 2009, all the navigation, corresponding markings, and firing systems to runway 26L were removed.
The acronyms HALS / DTOP stand for High Approach Landing System and Dual Threshold Operation.
— de.wikipedia via Google Translate [edited]



Special thanks to J. Hougaard and mins, they basically answered the question.