SAML - can't add certificate

I found a log-item in the control-panel docker that may give some insight.

When ask to self-generate:

error: http_://onlyoffice-community-server/sso/generatecert Unexpected token < in JSON at position 0

When loading a private key + certificate:>

2023-04-14 10:32:43 - error: http_://onlyoffice-community-server/api/2.0/portal.json
2023-04-14 10:32:43 - error: http_://onlyoffice-community-server/api/2.0/settings/controlpanel.json
2023-04-14 10:32:43 - error: http_://onlyoffice-community-server/api/2.0/people/@self.json
2023-04-14 10:32:43 - error: http_://onlyoffice-community-server/api/2.0/settings/controlpanel.json
2023-04-14 10:32:43 - error: http_://onlyoffice-community-server/api/2.0/settings/companywhitelabel.json
2023-04-14 10:32:43 - error: http_://onlyoffice-community-server/api/2.0/people/@self.json

Still no idea how to approach this. Seemingly generatecert gets a json that contains a “<”, but no idea how to debug that.