[nwh]

They change completely on every load. Encoding would likely have similar sections between page refreshes, as it doesn't we can assume your theory of encrypted data is correct. It makes sense anyway, it means they can have a user supply a stack trace or similar without exposing any information to them.

[aeon10]

Actually the first few characters remained the same with page loads.

[nwh]

CBC mode AES?

[drdaeman]

More likely a header. It's 3081 byte blob after base64 decoding. If we consider it's encrypted with some block cipher with 128 or 256-bit blocks (IV and/or MAC would be likely to be 128-bit, too), there are 9 bytes for some header and/or padding.

[nwh]

Good catch.