Boot into Windows partition, and use the free, non-commercial version of EasyBCD to reinstall the Windows bootloader.
Reboot and check that you're not using grub anymore.
Delete the Ubuntu partition and extend the Windows partition using the Windows Disk Management tool. Right-click the volume next to the empty space, and select Extend Volume to increase ...
Just a side note here:
Since Ubuntu version 17.10, the minimun version of the CIFS/samba protocol has been changed for security and performance reasons. Unfortunately, that breaks the network browsing function (which requieres version 1).
Taken from a discussion in Ubuntu Mate Forum.
And it hasn't been fixed yet, at least on Ubuntu 18.04 LTS.
I just encountered a similair problem on two(!) of my labtops (Dual booting with Win7 and Ubuntu 18 on Lenovo Thinkpad L450).
It occurred immediately after applying the "December 10, 2019—KB4530734 (Monthly Rollup)" - or actually during the intermediate reboot that Windows requires
I am positive that the root cause is the exact same thus it can be fixed ...
Fix was going ahead with Installation type and choosing Something Else, loading Ubuntu on last partition of drive. Able to boot Ubuntu and Windows 7. Ubuntu can also access file system on the "windows" partition. I also want to thank all those who gave me guidance on this issue. It has been a LONG time since I had to mess with volumes,partitions, Boot ...
Ignore the error, USB should still boot fine.
It turns out the USB drive was successfully written. Only some cleanup process crashed causing further writes on the same session to crash as well.