I’ve the same.
I’ve installed WP ERP (new and fresh WP installation 4.9.5) and all seem to work fine but not a one little thing.

None of the Activities are saved whether it is an email task or note or Schedule.
When pressing save/send a blue status bar starts running really, really slowly just below the black menu tabs on the right but that’s it.
Nothing happens.
How can I resolve this?