Eugene,
I know I had a similar challenge with my ISP for a domain I was attempting to validate. What I determined was the "TXT" record entry that is required for the check contains what essentially amounts to as a time stamp that the Office365 servers look for to validate the domain ownership. If this record gets too old, it will not pass the validation test and hence the "can't verify" result you are getting. I think the time between the "TXT" record submittal and the verify check is relatively short. I'm not sure on the exact time is, but I know that I could verify my domain after I resubmitted my ISP request and re-ran the check if updated within 12-24 hours.
Hope this helps in your specific situation.
George
