I had the same problem and it's with Resellerclub and WHMCS. Try registering the domain inside their "supersite" and you get the same error.
In my case it was an existing client in WHMCS who didn't have a customer record at Resellerclub. When WHMCS created the user record at RC, it had "0" for the address line 2 field, and since the length of "0" is 1, RC threw an error. I set address line 2 to blank and everything worked.
My guess is that when WHMCS was creating the record, it got an error somewhere and stored (int) false into line 2.
The validation error probably comes from the .CA registry, which is why it works for the .com and not the .ca. CIRA is really picky about valid data.