The debug information contained in the (urlsafe-)base64 blob is likely encrypted.
Think about it from Google’s perspective: You would want to display a stack trace, relevant headers of the http request and possibly some internal state of the user session to help a developer debug the situation. On the other hand all that information might contain sensitive information that you don’t want the general public to see or that might endanger the user if he copy’n pastes it in a public support forum.
If I was to take a guess of the format I would imagine:
- A public identifier of the key used for encryption (their servers could use different keys then)
- The debug data encrypted using an authenticated encryption scheme
- Additional data for error correction when OCR has to be used
For statistical analysis of the format it would be interesting to sample a lot of these error messages and see if some parts of the message are less random than you would expect from encrypted data (symmetrical encrypted data should follow a uniform distribution).