iOS 11.1.1 BREAKS charging from iMac on iPhone 6s

Thank you Giulio. I'm not new to tech and I've dealt with too many others whose reply is typically, "Try turning it off and back on, again." I become rather sensitive, if not defensive to such ultra-basic advice. Yours was a fair question and an elaboration on the things I tried does make sense for people offering assistance. I'd not put specific trouble-shooting steps tried in my original post.

I just used the same cable in an Apple wall adapter. It started charging normally. Green battery icon, lightning bolt icon beside it, and no error message. Using the very same cable again, I replugged in to my iMac. The error message came up in about 6 seconds. That rules out the cable. However, your wall plug power adapter trouble-shooting suggestion caused me to try something else.

I plugged in to a USB port on the back of the iMac itself. Prior attempts were from a powered USB hub because I have too many USB devices for the limited number of ports on the iMac. The error did NOT reoccur.

On closer inspection of the USB Hub, it appeared that the 3V DC power adapter had pulled out of the hub. All power used by the hub was being supplied by a USB port from the iMac. That low power is split between four total devices, including the iPhone. After re-"powering" the hub (simply restoring the adapter plug into the jack), I tried the iPhone USB cord in the hub. The error did NOT reoccur.

The iPhone seems to now be charging as expected with no error, plugged into the USB hub, as long as it has external power.

Thank you for giving ideas for further trouble-shooting. There is still a puzzle, though not quite as annoying as before. Prior to the iOS update, the iPhone powered fine in the [unpowered] USB hub. After the update it did not. Restoring adapter supplied power to the hub caused it to charge normally. Somehow, whether in the recent iMac operating system update, or the iOS iPhone update, slightly more power is now required for iPhone charging. Not the same amount as required by an iPad, but more than the unpowered hub was able to provide.

Now that I know this new power requirement, I have no further concerns or issues (at least not yet!).

Thanks, again for the suggestion to keep trouble-shooting, and the inspiration to dig further into the problem.

Alright, let's not be patronizing here. I KNOW how to read instructions and I KNOW what to do.

First, the setup leading up to the update: I was successfully connected to my iMac, backed up my iPhone, the power was charging, and NO issues PRIOR to the iOS update.

Second: I followed the iTunes link for iOS update to 11.1.1 notice.

Third: The iPhone downloaded and updated the iOS. As per the process, the iPhone rebooted. ALL this time the iPhone stayed connected to the iMac.

Fourth: The iPhone came back on. I entered my ID on the phone. Immediately the message "This accessory may not be supported." appeared. I tapped "Dismiss," the only option given.

Fifth: I detached the USB charging cable from the iPhone and plugged it back in again. Same message.

Sixth: I conducted a search of the message both on the internet AND in Apple support forums.

Seventh: I verified the phone did not have "debris" in the charging port (since it had already been connected with no issue BEFORE the update, this was actually a non-issue). The cable is the cable that came with my iPhone and is NOT damaged. Then I followed the suggested process of : Switch the phone to Airplane mode - check. Power off the phone and wait 1 minute - check. Power the phone back up - check. ERROR OCCURS AGAIN.

Eighth: Unplug the USB cable and repeat the process in "Seventh" above. After power-up, replug the USB cable in the phone. ERROR OCCURS AGAIN.

Ninth: Try it all one more time. ERROR OCCURS AGAIN.

Tenth: Conducted more online research of both internet and Support forums. No other processes OTHER than filing a service request was offered. SINCE I was already successfully connected (see "First" above), my phone is NOT in need of servicing. Thus the support forum entry.

CONCLUSION: There was ONE change preceding the error message and failure to charge from the USB port of my iMac. That ONE change was updating to iOS 11.1.1. Nothing else. Oh, and yes. I DID try another Apple issued USB cable. Same result.