I had the same issue in the past. BTRFS CoW and Win VMs do not play well together. You can search the web and find several examples of similar errors (with nice technical explanations why that happens). Just disable CoW for the whole share you use for VM files, or for the single files. For instance, in my case, the Linux VMs were not creating any issue, only the Win one. Unfortunately I cannot tell you the commands I used because I have lost them. Once I have disabled CoW for the Win VM, all the errors disappeared.