So, I also had this problem, a change in my stray file was fixed for me:
config.vm.synced_folder "C:/dev/vm/share", "/var/www/", mount_options: ['dmode=777','fmode=666']
to
config.vm.synced_folder "C:/dev/vm/share", "/var/www/", mount_options: ['dmode=777','fmode=777']
There are many tips saying that 666 is permissible, but in my case it is not, and since it is only a development machine, the security implications are not very important.
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