I too upgraded my HTC One and cannot see text messages any longer. Started fresh, disconnected link to the phone in the car and vice versa on my phone, as if it were a new phone, and when I hit the button for text messages I get a "text messages not available at this time". Anyone have any ideas? Not sure what is meant by "sideloading", I just erased the connection on phone and car and paired it like a new phone, but texts don't sync.
Sideloads means to install Apps or in this case the OS manually using a Computer or uploading the App or OS to any kind of Memory Storage on your phone, and then installing it manually.
So, in my case, none of the OTA updates since I have the phone (April 2013) have installed, I've needed to installed all of them manually, on this instance, doing it, all my settings and apps were deleted, so I needed to Installed everything and all my settings back.
Like I mentioned before, that's the only thing different, but I don't know, why some of you may have this situation. Seems weird, maybe some kind of permission issue on your phone, if it began just after updating your phone, then with the new connection established and connected, check your phone's connection on the Bluetooth menu to see if you can see some kind of permission or setting related. If not, I'd tell you to delete the connection again in both, and creates it again, just pay very close attention to every single message on your phone when it tries to establish it, leave it do it's job, it takes a couple of seconds after establishing the connection the first time to ask you about different features that it needs access on your phone, such as Contacts and History, click yes and check the Always remember option.
Is not that I think you don't know to do it, it's obvious you do because it was working before, I'm just saying that it maybe possible that recreating the link between them some steps may have been skipped and it's causing your issue. Anyways, Good luck.