MTP - Can't copy files to device

Since I upgraded my Z3 Compact to Lollipop I'm unable to copy files or folder from my PCs to the device.
I plug in the USB cable, it informs me that it's connected in MTP mode. When I open explorer in Windows I can

• Copy data from device to PC.
• Delete files on the device from Explorer.
• Create Folders on the device from Explorer.
• Copy Folders to the device from Explorer.
• But I CANNOT copy any actual files to the device.

I've tried the usual things, different USB cables/ports, various PC and OS variants. No joy on Win XP, 7, 8, 8.1, 10. Tried installing PC companion software, various versions of drivers and nothing seems to work. I don't have an SDCard installed (don't ordinarily need it or have one) so I can't use MSC mode.

It's incredibly irritating that something so basic no longer works. Anyone have any ideas that don't involve going back to 4.4? Thanks.

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