Hi,
I think I also have seen lockups with digiKam edited-XMPs and then darktable trying to reload the XMP content. But currently with my setup (digiKam 7.0.0 and darktable master darktable_3.1.0~git2511.4226ac17c_amd64 (running in MX Linux - Debian 10based) I do no longer see the lockups. Might be taht there have been done chenges in DT/digiKam to enhance XMP interoperability.