It fixed itself in about 20 minutes but I send note about it to our service provider anyway and it's being looked at. Apparently it wasn't just us as few others replied to my topic in the help forum saying they saw the same message. To make things clear: no one outsider has visited admin panel, all messages seem to be in their place and overall everything seems to be normal now. But I recommend changing passwords just to be safe. I'll update you all if I find out more
