After reading the help page for engine-setup and discovering that the option "--reconfigure-optional-components" has no effect in 4.1 for Image I/O Proxy, I used the workaround offered throughout this bugzilla:

Now it seems all ok, but the upload fails after going into pause just at the beginning with the message

"

Unable to upload image to disk de28015a-39e9-44e4-acb2-2d2e3b9cdc7f due to a network error. Make sure ovirt-imageio-proxy service is installed and configured, and ovirt-engine's certificate is registered as a valid CA in the browser. The certificate can be fetched from https://<engine_url>/ovirt-engine/services/pki-resource?resource=ca-certificate&format=X509-PEM-CA

"

I already fetched into firefox the certificate and selected all 3 check boxes when configuring...

Any other thing to check?

On engine:

[root@ovirt ~]# firewall-cmd --list-all
public (active)
  target: default
  icmp-block-inversion: no
  interfaces: eth0
  sources:
  services: ovirt-websocket-proxy ovirt-vmconsole-proxy ovirt-http dhcpv6-client ovirt-https ssh ovirt-postgres ovirt-fence-kdump-listener ovirt-imageio-proxy
  ports:
  protocols:
  masquerade: no
  forward-ports:
  source-ports:
  icmp-blocks:
  rich rules:
   
[root@ovirt ~]# 

[root@ovirt ~]# systemctl status ovirt-imageio-proxy.service -l
● ovirt-imageio-proxy.service - oVirt ImageIO Proxy
   Loaded: loaded (/usr/lib/systemd/system/ovirt-imageio-proxy.service; enabled; vendor preset: disabled)
   Active: active (running) since Wed 2017-12-06 00:07:48 CET; 7h ago
 Main PID: 9402 (ovirt-imageio-p)
   CGroup: /system.slice/ovirt-imageio-proxy.service
           └─9402 /usr/bin/python /usr/bin/ovirt-imageio-proxy

Dec 06 00:07:47 ovirt systemd[1]: Starting oVirt ImageIO Proxy...
Dec 06 00:07:48 ovirt systemd[1]: Started oVirt ImageIO Proxy.
[root@ovirt ~]#

In  /var/log/ovirt-imageio-proxy/image-proxy.log

(MainThread) INFO 2017-12-06 00:07:48,460 image_proxy:26:root:(main) Server started, successfully notified systemd

In engine.log

2017-12-06 00:10:07,534+01 INFO  [org.ovirt.engine.core.bll.storage.disk.image.ImageTransferUpdater] (DefaultQuartzScheduler1) [cefb309e-1127-4811-b69b-a7d83c3f7df6] Updating image upload 208cc8c5-66ae-40aa-9748-04ee90c022a6 (image bad43962-dcc4-4f16-8b8e-dafc26573e2c) phase to Transferring (message: 'Initiating new upload')
2017-12-06 00:10:07,537+01 INFO  [org.ovirt.engine.core.bll.storage.disk.image.TransferDiskImageCommand] (DefaultQuartzScheduler1) [cefb309e-1127-4811-b69b-a7d83c3f7df6] Returning from proceedCommandExecution after starting transfer session for image transfer command '208cc8c5-66ae-40aa-9748-04ee90c022a6'
2017-12-06 00:10:10,506+01 INFO  [org.ovirt.engine.core.bll.storage.disk.image.TransferImageStatusCommand] (default task-1) [dae9a850-5db2-40be-92ca-a6fff0d54aac] Running command: TransferImageStatusCommand internal: false. Entities affected :  ID: aaa00000-0000-0000-0000-123456789aaa Type: SystemAction group CREATE_DISK with role type USER
2017-12-06 00:10:14,512+01 INFO  [org.ovirt.engine.core.bll.storage.disk.image.TransferImageStatusCommand] (default task-15) [ca329503-0916-43f4-99a7-e285775d5ffd] Running command: TransferImageStatusCommand internal: false. Entities affected :  ID: aaa00000-0000-0000-0000-123456789aaa Type: SystemAction group CREATE_DISK with role type USER
2017-12-06 00:10:18,512+01 INFO  [org.ovirt.engine.core.bll.storage.disk.image.TransferImageStatusCommand] (default task-3) [66f4e540-670e-4e20-8003-4e740ee8c3b6] Running command: TransferImageStatusCommand internal: false. Entities affected :  ID: aaa00000-0000-0000-0000-123456789aaa Type: SystemAction group CREATE_DISK with role type USER
2017-12-06 00:10:20,442+01 INFO  [org.ovirt.engine.core.bll.storage.disk.image.TransferImageStatusCommand] (default task-16) [403dd7c1-9f2b-4d26-8fae-f179ca1922e1] Running command: TransferImageStatusCommand internal: false. Entities affected :  ID: aaa00000-0000-0000-0000-123456789aaa Type: SystemAction group CREATE_DISK with role type USER
2017-12-06 00:10:20,443+01 INFO  [org.ovirt.engine.core.bll.storage.disk.image.ImageTransferUpdater] (default task-16) [403dd7c1-9f2b-4d26-8fae-f179ca1922e1] Updating image upload 208cc8c5-66ae-40aa-9748-04ee90c022a6 (image bad43962-dcc4-4f16-8b8e-dafc26573e2c) phase to Paused by System (message: 'Sent 0MB')
2017-12-06 00:10:20,461+01 WARN  [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (default task-16) [403dd7c1-9f2b-4d26-8fae-f179ca1922e1] EVENT_ID: UPLOAD_IMAGE_NETWORK_ERROR(1,038), Correlation ID: 403dd7c1-9f2b-4d26-8fae-f179ca1922e1, Call Stack: null, Custom ID: null, Custom Event ID: -1, Message: Unable to upload image to disk bad43962-dcc4-4f16-8b8e-dafc26573e2c due to a network error. Make sure ovirt-imageio-proxy service is installed and configured, and ovirt-engine's certificate is registered as a valid CA in the browser. The certificate can be fetched from https://<engine_url>/ovirt-engine/services/pki-resource?resource=ca-certificate&format=X509-PEM-CA

Thanks,

Gianluca