Keychain error when restoring from backup

My phone (iPhone 6S Plus, 128GB) has been seriously acting up recently so I finally caved and reset to factory settings, as suggested. When trying to set it up again however, from a backup on my Mac (MacBook Pro, 2.9 GHz Intel Core i7, 8 GB 1600 MHz DDR3) from just before the reset, I run in to problems with Keychain. No matter what password I put in, in order to get the restore going, I'm told it's incorrect. I've tried every possible password since getting my Mac - a five year period - to no avail. Is there anything I can do to fix this? My phone is basically a fancy brick right now.

I did a restore from backup earlier this year with no problems, I'm not sure what could have changed.

I'm running macOS High Sierra (10.13.2 Beta) and iOS 11.1.1