There is no generic plain-text serializer, the response type would only be used when returning a text body.
I’ve updated the event unregister API to force returning a JSON response in this commit which is now available from the latest v5.10.5 that’s now available on MyGet.
But it should be returning the default JSON ContentType, do you have features disabled in your AppHost configuration?
This is the normal Request/Response of calling /event-unregister
Request
POST /event-unregister?id=dGVh0F82gtyz3M3QC2T4 HTTP/1.1
Host: chat.netcore.io
Connection: keep-alive
Content-Length: 0
Accept: */*
DNT: 1
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.150 Mobile Safari/537.36
Origin: http://chat.netcore.io
Referer: http://chat.netcore.io/
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Cookie: ss-pid=VGdSVvVx9nQslRMNBxTf; ss-id=GfJUhb3uZ5R0MffRK6SA
Response
HTTP/1.1 200 OK
Server: nginx/1.17.5
Date: Fri, 19 Feb 2021 09:51:27 GMT
Content-Type: application/json; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
Vary: Accept
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: Content-Type, Allow, Authorization
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, PATCH, OPTIONS, HEAD
X-Powered-By: ServiceStack/5.104 NetCore/Linux
{"userId":"-13","isAuthenticated":"false","displayName":"user13","channels":"home","createdAt":"1613728281284","profileUrl":"data:image/svg+xml,%3Csvg width='100' height='100' viewBox='0 0 100 100' xmlns='http://www.w3.org/2000/svg'%3E %3Cstyle%3E .path%7B%7D %3C/style%3E %3Cg id='male-svg'%3E%3Cpath fill='%23556080' d='M1 92.84V84.14C1 84.14 2.38 78.81 8.81 77.16C8.81 77.16 19.16 73.37 27.26 69.85C31.46 68.02 32.36 66.93 36.59 65.06C36.59 65.06 37.03 62.9 36.87 61.6H40.18C40.18 61.6 40.93 62.05 40.18 56.94C40.18 56.94 35.63 55.78 35.45 47.66C35.45 47.66 32.41 48.68 32.22 43.76C32.1 40.42 29.52 37.52 33.23 35.12L31.35 30.02C31.35 30.02 28.08 9.51 38.95 12.54C34.36 7.06 64.93 1.59 66.91 18.96C66.91 18.96 68.33 28.35 66.91 34.77C66.91 34.77 71.38 34.25 68.39 42.84C68.39 42.84 66.75 49.01 64.23 47.62C64.23 47.62 64.65 55.43 60.68 56.76C60.68 56.76 60.96 60.92 60.96 61.2L64.74 61.76C64.74 61.76 64.17 65.16 64.84 65.54C64.84 65.54 69.32 68.61 74.66 69.98C84.96 72.62 97.96 77.16 97.96 81.13C97.96 81.13 99 86.42 99 92.85L1 92.84Z'/%3E%3C/g%3E%3C/svg%3E"}
I’m assuming something is off in your AppHost configuration or Ajax request, what API are you using to unregister and what does your HTTP Request/Response look like?
From the sounds of it, it’s explicitly requesting a plain/text response which would be expected to fail to serialize (though the unregister operation would still have been executed).