The link below will take you to Dual-Lite's "Somewhat Common" Fluorescent Battery Backup Ballast Wiring Schematic (PDF File):UFO-3AW
Schematics on the last page (page 4).
There are Two Ungrounded Leads for the Battery Backup Ballast: "Switched Power" and "Unswitched Power".
I have a few questions to ask...
1: The Voltage Measurements you posted, were these taken L-G (Line to Ground), or L-N (Line to Neutral)?
2: Is there a readable Voltage between the Two Ungrounded Conductors feeding the Ballast (at the Fixture)?
3: Which Circuit is feeding the Em. Battery Backup Ballast's "Unswitched Lead"
the "115 Volts" Circuit, or the "90 Volts" Circuit?
4: If you turn off the Switch(es) controlling the Lights on the effected Circuit, does the Voltage on the "Unswitched"
Circuit at the Em. Ballast change?
5: What is the length of the Switched Circuit? (from Panelboard to Fixture you are Shooting Trouble at),
6: What is the FLA (Full Load Amperes) of the effected Circuit - measured at the Branch Circuit OCPD?
7: What is the Voltage between N-G (Neutral to Ground) at the Fixture with the Switched Fixtures:
The issue appears to be poor makeup in One or more Fixtures.
Could be Open Common Neutral, Fried Wirenut(s), incorrect connections at Fixture(s), Open Neutral to Panelboard - Grounded at One (or more) Fixtures...
Or maybe a Voltage Drop issue from too many Fixtures on same Circuit.
Will wait for your reply.