I faced the same problem, however it was not due to refunding with a different currency. The issue was with the text limit of 255 characters in the "Note to Buyer" text box. There is no alert (at least with Chrome browser) if you go slightly over this limit and once you click send then the dreadful message "We can't process any refund at this moment due to technical reasons. Please try again later." appears. Going over the limit will not allow you to send the refund. Unfortunately it just shows a general error and not an error to suggest that you went over the text limit of 255 characters. I discovered this error as I tried to do the refund with a different browser. Then an alert popped up when I pasted the exact same text in the "Note to Buyer" field that I was over the 255 characters limit. The refund went through once I reduced my note to below 255 characters. So make sure your "note to buyer" stays below the 255 characters limit.
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