It seems to be related to the new XSRF checks. Possibly that is returning 403 when it fails and not a more accurate message in some cases. I've only seen it with some rest end points provided by a plugin, haven't been able to reproduce with a confluence endpoint. There is no error in the confluence log.
Example 5.8.10:
curl -D- -u admin:admin -X POST http:HTTP/1.1 415 Unsupported Media Type
Server: Apache-Coyote/1.1
Set-Cookie: JSESSIONID=08E571811595069B2EC57B68BFEC6FDB; Path=/
X-Seraph-LoginReason: OK
X-AUSERNAME: admin
X-Content-Type-Options: nosniff
Content-Type: text/html;charset=utf-8
<html><head><title........
5.9.1-pluginsfour013
curl -D- -u admin:admin -X POST http:HTTP/1.1 403 Forbidden
Server: Apache-Coyote/1.1
Set-Cookie: JSESSIONID=BA414C125E8008AADA95281DE15E05D3; Path=/confluence/; HttpOnly
X-Seraph-LoginReason: OK
X-AUSERNAME: admin
X-Content-Type-Options: nosniff
Content-Type: text/html;charset=ISO-8859-1
XSRF check failed
It seems to be related to the new XSRF checks. Possibly that is returning 403 when it fails and not a more accurate message in some cases. I've only seen it with some rest end points provided by a plugin, haven't been able to reproduce with a confluence endpoint. There is no error in the confluence log.
Example 5.8.10:
curl -D- -u admin:admin -X POST http://localhost:1990/confluence/rest/cw/latest/workflows/ds/workflowParameter HTTP/1.1 415 Unsupported Media Type Server: Apache-Coyote/1.1 Set-Cookie: JSESSIONID=08E571811595069B2EC57B68BFEC6FDB; Path=/ X-Seraph-LoginReason: OK X-AUSERNAME: admin X-Content-Type-Options: nosniff Content-Type: text/html;charset=utf-8 <html><head><title........
5.9.1-pluginsfour013
curl -D- -u admin:admin -X POST http://localhost:1990/confluence/rest/cw/latest/workflows/ds/workflowParameter HTTP/1.1 403 Forbidden Server: Apache-Coyote/1.1 Set-Cookie: JSESSIONID=BA414C125E8008AADA95281DE15E05D3; Path=/confluence/; HttpOnly X-Seraph-LoginReason: OK X-AUSERNAME: admin X-Content-Type-Options: nosniff Content-Type: text/html;charset=ISO-8859-1 XSRF check failed