[TX Builder] Undocumented batch size limit leading to generic error

[d10r]

When trying to import this json file with 200 txs into the tx builder, initially everything seems to work as expected:

Simulation succeeds:

I can provide a signature for creating the batch:

But after providing the signature, it fails with 500: internal server error:

With 100 such nested txs, it reliably succeeds.
There seems to be a hidden limit hit somewhere between 100 and 200 such transactions.