What is the scientific support for Einstein's claim about the negative effects of reading too much?
This answer is a bit more anecdotal, but perhaps it's useful.

From the perspective of an academic researcher (which perhaps is similar to Einstein's perspective), there is a balance between reading the literature and conducting your own research. There is a balance between learning new skills and applying those skills to your own projects. Even within the context of learning, there is a balance between reading how something is done and practicing actually doing it.

The very process of conducting your own research reveals challenges and gaps in understanding. Reading allows you to take on new information, however, doing good research also requires time to re-arrange ideas, refine your own theories. Furthermore, reading can vary in the degree to which it is a passive or active process. As an active process, choosing what to read, critically evaluating reading, is all part of building knowledge and ideas.

These ideas manifest in many places in curriculum design. A PhD thesis in particular is a good example of a project that requires a balance between understanding the existing literature and independent thought.

I presume that the quote is just a useful linguistic device to get people to think about this balancing process.