I'm experiencing the same problems very often lately with several client (backup over Internet). I got Async indexing process not found both on windows and linux client. The problem happens randomly: sometimes the client return this error, sometimes it's able to work fine.
My server is versione 2.1.19 working on Ubuntu 16.04, my clients are 2.1.16. I read what you wrote, are these releases patched?
What could I do to send you some relevant information in order you can discover if there is a bug?