I see there's a a question asked by Rafael back in May, but no resolving answer to the question.
I'm having the same issue on version 10.2.8.0-12005 of DataMiner.
The element doesn't have bypassproxy in the bus address, and has stopped working in protocols I have used it in before, so looks like a core software bug.
There are currently no known issues regarding the proxyServer tag attribute and I double checked our test setups running DataMiner 10.2.10 where it seems to work as expected.
The only reason I can think of why requests would not pass the configured proxy server would be if bypassproxy is configured on the interface. Can you double check with a search in the Element.xml on disk to verify there's definitely no such thing configured?
If there's not, would it be possible to take an export of the element and send it to support.data-acquisition@skyline.be. We'll be able to do a more in depth investigation from there and will come back here with more information when available.