OPManager Interface Graphs Pictures not displayed sometimes

OPManager Interface Graphs Pictures not displayed sometimes

We use OPManager 7023 with a few cisco firewalls.
But we can only see the Interface traffic Image sometimes.

In most cases the image is missing and a red cross is displayed (see attachment).
We deleted and readded the firewalls several times but it is always the same.
Sometimes you can see the traffic interafce graph and a few hours later it doesn't work.

Thanks in advance.






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