In no particular order, I'll add some observations.

If GTID is used binlogrouter will use 'binlogdir'/gtid_domain_id/server_id for binary logs, while avroservice will still look into root 'binlogdir', in such case it's needed to use the full path in the avroservice parameter.

If the directory set in 'binlogdir' doesn't exist, the message is misleading at least:

2018-09-11 14:29:09   error  : Unexpected parameter 'binlogdir' for object 'avro-service' of type 'service', or '/tmp/maxscale/0/10133' is an invalid value for parameter 'binlogdir'.

I have the impression that different exceptions have been joined into one with the result of some nonsense for the user.

i) "Unexpected parameter 'binlogdir' for object 'avro-service' " it's false, the parameter exists and it's perfectly placed.
IMHO this message should be removed when the problem is a non existing directory, or generally speaking I would not group such different error messages into the same exception(if that is).

ii) "'/tmp/maxscale/0/10133' is an invalid value for parameter 'binlogdir'." it's technically true but invalid makes me think something is wrong with the format, not the value itself.
More meaningful message would be "Path not found" or similar.