Hi all,
We just rushed out an urgent hotfix release for Synapse 0.26.1, addressing a nasty bug in the ujson library which causes it to misbehave badly in the presence of JSON containing very large >64-bit integers. Anyone whose synapses are currently filling up with "Value too big!" errors will want to upgrade immediately from https://github.com/matrix-org/synapse/releases/tag/v0.26.1.
Sorry for the inconvenience.