[FS#742] Cannot "upgrade" from OpenWRT 15.05.1 to LEDE 17.01.1

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Uqbar (Uqbar) 

Attached to Project - LEDE Project
Summary - Cannot "upgrade" from OpenWRT 15.05.1 to LEDE 17.01.1
Task Type - Bug Report
Category - Documentation
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - High
Priority - Very Low
Reported Version - lede-17.01
Due in Version - Undecided
Due Date - Undecided
Details - Device: TP-Link Archer C7
Coming from: OpenWRT 15.05.1
Going to: LEDE 17.01.1

I have done a "sysupgrade" and I have got non working web UI (LuCI).
Most of the time the buttons were reporting:

The submitted security token is invalid or already expired!
In order to prevent unauthorized access to the system, your request has been blocked. Click "Continue »" below to return to the previous page.
Continue »


The "fix" has been to reflash a "factory" image for LEDE with no setting left over and to reconfigure everything from scratch.
I had no chance to try a settigns restore over the factory reflash.
I'd say this needs to be documented.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=742