What are the options for upgrading the inverter for 48V system where the system was designed to use 48V because of shading reasons? That is, it is not feasible to serially connect all modules into a high-voltage strings because a number of panels within a given string may be subject to severe shading at different times of day.

I have looked at Outback, but they don't offer a simple grid-tie solution.

None that I know of staying at 48V ... the UPG is the best available inverter in the 48V gridtie performance, about 3% better than using an outback/mx60 and the UPG is true continuous mppt tracking vs the MX60's sweep and sleep method

If your having issues with the UPG, I'd see if you can get xantrex to fix it.

Your other option is to series all the panels, as long as the worst case shading keeps the vmp above ~200 volts, having shaded panels would not really matter much as the solar panels bypass diode kick-in.

So best bet for an upgrade is to have a SINGLE series string with a GT or similar string inverter