This problem seems to be not yet fixed.
My mailbox at gmx.com (which stopped receiving emails forwarded from Opalstack on 29th December) still has not resumed receiving them.
Please note that gmx.com and gmx.net are distinct entities (the former internationally oriented and the latter more Germanocentric) with different account holders and slightly different services.
Could it be that only gmx.net has unblocked Opalstack while gmx.com is still blocking it?
As a sanity-check I have (ten minutes ago) added a mailbox at gandi.net as an extra new forwarding address for my Opalstack emails, and this has started receiving forwarded emails immediately.