Hello!
Besides painting 3 seals, you'll have 5 soul points consumed to pass through the portal. You'll receive this message: "Venturing forth into Tibia's past only costs you a small part of your soul. Go for it!".
So if you have 4 or less soul points, your passage'll be blocked. If that's not the case, I suggest you to open a bug report, since it may be just some issue that may need fix.
You can check the spoiler here