Sony Z3 can't transfer files by USB and isn't recognised by PC after update to 6.0.1

Question

My Sony Xperia Z3 Compact was recently updated to android version 6.0.1 ("Marshmallow"?) using the Sony update tool, from version 5. Phone isn't rooted, no custom stuff, only ever updated via Sony updates, etc. UK based.

Before the update, if I connected it by USB to my (Windows 8.1) laptop, the following would happen:

The SD card would appear on the PC as if it was USB pen drive

The phone charges and I'd get a message on the phone asking if I wanted to "install" Sony software on the PC (every time, even if I'd agreed to "install" it last time)

On saying Yes, the SD card would disappear and would be replaced by a Sony icon that let me access both the internal memory and SD card like they were two directories on a USB pen drive

After the update, the following happens. Basically I can never access the SD card or internal memory on the PC no matter what I choose:

The PC doesn't react, nothing appears on the PC

The phone charges and a menu pops up giving me four choices:

Use USB for

Charging only Just charge this device

Transfer files (MTP) Transfer files to Windows or Mac (MTP)

Mass storage mode (MSC) Transfer data between phone SD card and a computer

MIDI Use device for musical instrument apps (MIDI input)

Neither "Transfer files (MTP)" nor "Mass storage mode (MSC)" do anything when selected. If I select either of these, the menu disappears, but nothing else happens - nothing appears on the PC (no drives or anything), nothing displays on the phone.

It behaves exactly the same as if I'd selected "Charging only".

How can I investigate why this isn't working? What do I need to do to make file transfer on 6.0.1 with a PC by USB work like it used to? Is there some special Sony-only thing I need to do?

I've looked at Marshmallow USB connection type. Various answers there suggest things like:

Swipe down the notifications window with the device plugged into the computer, there should be a persistent notification for USB Mode

...there actually isn't any such persistant notification at all for me, no matter which mode I choose. Also, unlike the description of that question:

Windows detects it, installs the drivers automatically and shows the device in My Computer...

...none of that happens for me. The computer is completely unaware of the phone, although the phone does react to being connected to the computer.

— user56reinstatemonica8 | Sources

Answer

This is bizarre. I connected the phone via a different USB socket on the computer (same cable), and this time it worked fine, in fact better than before:

The Sony icon appeared without needing to "install" anything, and accesses both internal and SD memory like before

The "Install software" Sony message pops up on the phone like before now

The missing persistent notification on the phone appears:

USB for file transfer Touch for more options

I wasn't aware of the computer having different types of USB socket, but it seems like maybe there are some types of USB socket that an Android 6 phone will think work but actually don't?

I thought it might maybe have been a driver install thing, but the "bad" USB socket still doesn't work even after successfully using the connection to the "good" socket.

To add to the mystery, I have actually used this "bad" USB socket on this computer with this phone before, mere days before the Android update. Somehow the USB socket appears to compatible with Android 5 but not Android 6?

The "bad" USB socket also has no problem working with USB pen drives.

If anyone happens to know how I can find out what the technical difference between the two identical-looking USB sockets on my PC is, please comment!

— user56reinstatemonica8 | Sources

Sony Z3 can't transfer files by USB and isn't recognised by PC after update to 6.0.1

Use USB for

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