Quick follow-up as promised:
Checking for broken libraries in my installation and build directories like @Wile_E_Coyote proposed shows the exact same behavior as stated in his report.
The check also shows the issue is not in any way connected to missing/broken (file) links as I naively expected, but rather is due to the libraries themselves being broken.
I proposed a crude fix here:
Installation of ROOT to a user-defined prefix results in broken libraries for external packages (R, cfitsio)