System: 3VSB voltage in enclosure Internal is out of spec

3VSB (3 Volts StandBy) is really 3.3V, so your 2.96V would be out of spec.  That is, in turn, derived from the 5VSB from the power supply, which is a separate voltage that's on all the time unless the unit is unplugged or the rear switch is off, so it can be used to wake the NAS via the power button or WoL.  It is often what fails on the power supply due to age, but is especially sensitive if the NAS has been allowed to get to warm when it's "off".

There was no change in the limits in the ULTRA6.conf file in any recent OS update.  I don't have a log file for an Ultra6 running OS6, but boot_info.log in the logs .zip file contains the limits.  In the log I have from OS4.2.31, it has this:

labelvol7:	3VSB    
minvol7:	3.07
maxvol7:	3.53

Since nobody else has reported a similar issue, then the only way I can see the update causing it is that it's now notifying you of something that it previously did not, so the problem has really been there longer..  Take a look at boot_info.log from your unit now and from some time before the OS update and see if the limits for that voltage changed.  Note that I think the limits in OS6 are  3.135 to 3.465, not those listed above for OS4.2.31, but the important quiestion is whether or not they changed in the recent update you made.

Assuming the limits didn't change, the power cycle associated with the update may have been the cause.  So, the next question is whether the voltage really is too low or if the components that measure it have degraded and it's a false alarm.  And if it's really too low, is it the power supply or the components that produce 3.3V from the 5V?  That's really hard to assess other than by swapping out the supply, but there are potential other signs.

Are you seeing any issue with powering on or off, or with the LAN connection?  If so, it's really the voltage.  But that still doesn't differentiate between the supply and down-converter.  Since the supply is a common issue at this age of your unit, it's a good guess.  If you have a standard ATX supply, you can use it externally just as a test before you buy a replacement supply.