OAuth error in Advanced Monitoring following Plesk redirect rule

I recently added a Plesk redirect rule:

File: /etc/sw-cp-server/conf.d/z-plesk.inc
> if (host !~ 'hostname.com'){ > rewrite ^/(.*) https://hostname.com:8443/$1 permanent;
> }
as per https://support.plesk.com/hc/en-us/articles/115001421414

Following that, the Advanced Monitoring tool is unable to load any tab as Grafana consistently fails OAuth:
login.OAuthLogin(NewTransportWithCode)

Log file for grafana shows:
$ tail /var/log/grafana/grafana.log
t=2021-01-07T02:36:32+0000 lvl=eror msg=login.OAuthLogin(NewTransportWithCode) logger=context userId=0 orgId=0 uname= error="oauth2: cannot fetch token: 405 Method Not Allowed\nResponse: "
t=2021-01-07T02:36:32+0000 lvl=eror msg=“Request Completed” logger=context userId=0 orgId=0 uname= method=GET path=/login/generic_oauth status=500 remote_addr=127.0.0.1 time_ms=55 size=1768 referer=https://myredactedhostname.ca:8443/modules/monitoring/index.php/index/main
t=2021-01-07T02:36:33+0000 lvl=eror msg=login.OAuthLogin(NewTransportWithCode) logger=context userId=0 orgId=0 uname= error="oauth2: cannot fetch token: 405 Method Not Allowed\nResponse: "
t=2021-01-07T02:36:33+0000 lvl=eror msg=“Request Completed” logger=context userId=0 orgId=0 uname= method=GET path=/login/generic_oauth status=500 remote_addr=127.0.0.1 time_ms=58 size=1768 referer=https://myredactedhostname.ca:8443/modules/monitoring/index.php/index/main
t=2021-01-07T02:36:34+0000 lvl=eror msg=login.OAuthLogin(NewTransportWithCode) logger=context userId=0 orgId=0 uname= error="oauth2: cannot fetch token: 405 Method Not Allowed\nResponse: "
t=2021-01-07T02:36:34+0000 lvl=eror msg=“Request Completed” logger=context userId=0 orgId=0 uname= method=GET path=/login/generic_oauth status=500 remote_addr=127.0.0.1 time_ms=59 size=1768 referer=https://myredactedhostname.ca:8443/modules/monitoring/index.php/index/main
t=2021-01-07T02:36:35+0000 lvl=eror msg=login.OAuthLogin(NewTransportWithCode) logger=context userId=0 orgId=0 uname= error="oauth2: cannot fetch token: 405 Method Not Allowed\nResponse: "
t=2021-01-07T02:36:35+0000 lvl=eror msg=“Request Completed” logger=context userId=0 orgId=0 uname= method=GET path=/login/generic_oauth status=500 remote_addr=127.0.0.1 time_ms=62 size=1768 referer=https://myredactedhostname.ca:8443/modules/monitoring/index.php/index/main
t=2021-01-07T02:36:35+0000 lvl=eror msg=login.OAuthLogin(NewTransportWithCode) logger=context userId=0 orgId=0 uname= error="oauth2: cannot fetch token: 405 Method Not Allowed\nResponse: "
t=2021-01-07T02:36:35+0000 lvl=eror msg=“Request Completed” logger=context userId=0 orgId=0 uname= method=GET path=/login/generic_oauth status=500 remote_addr=127.0.0.1 time_ms=58 size=1768 referer=https://myredactedhostname.ca:8443/modules/monitoring/index.php/index/main

I have already tried to restart the grafana service,
re-install grafana
re-install advanced monitoring
… without much success.

Any ideas ?