Found it. Error is related to https://github.com/xapi-project/xen-api/issues/4141
Somehow, the PCIe bus address of the server's ASPEED VGA adapter changed.
I got the new bus address with lspci and edited state.db accordingly. Works now. However, this seems to be a very strange bug.
stormi created this issue in xapi-project/xen-api open XAPI failing to initialize after GPU removal? #4141