Here is the full story: I had installed tensorflow and keras using pip, and they had pulled their dependencies, which could not be satisfied with Debian system packages.
My theory is that Lollypop could not start because it could have been using some python packages as dependencies that had been installed using pip. Unfortunately I cannot pinpoint the offending python package. Now after I removed all pip-installed python packages, Lollypop starts normally. This bug can be closed now.
