growatt have removed support for third party batteries
That is a pain for us DIYers
Assume you can't downgrade the firmware on your Growatt to revert to a version that does support third party batteries?
What is initially strange is that AFAIAA Fogstar use Pace BMS which supports Pylontech protocol and you say Growatt support Pylontech too - so it is strange that that combination doesn't work.
I _suspect_ (and this is a wild guess) is the error is due to Growatt doing more than just using the Pylontech protocol. Pylontech batteries use a 15S architecture, rather than the more common industry standard of 16S, so will typically run 3.2V lower than a regular 16S battery. Hence Growatt may have implemented some code to detect voltages over (say) about 54V (corresponding to 15 cells @ 3.6V) and trigger an over-voltage warning if that happens.
Having said that the datasheet for the SPH3600 states battery voltage range 42-59V and support for LiFePO4 or Lead Acid.
Did you receive any assurances from Growatt or your supplier that the SPH3600 would work with generic LiFePO4 batteries? If so, maybe you can reject the product under CRA2015 rules or push them for a firmware downgrade to get it to work.
Apart from that, changing to a different inverter, for example, Solis might be the simplest solution. Or if you're into software (you refer to engineering in your profile) maybe a MITM CANBus/RS485 interface between the Growatt and Fogstart battery could translate the necessary data into a format both devices are happy with?