We'll update this thread as soon as we have more information to share. Last but not least, the maintenance for Diablo III and StarCraft II are not affected by this.
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Update: ETA is 13:30 CEST.
Update 2: Maintenance continues on all realms, we do not have an ETA for completion at this time. We're sorry for the delay on this update, the community sites are also experiencing problems. We thank you for your patience and understanding.
Update 3: We have brought the realms up a few minutes ago and everything is looking good. Thanks everyone for your patience and understanding!