I’ve seen a number of users having issues logging into the mobile app because they accidentally or through copy/paste add a space after their username.
Since the username is an email address I suggest you trim leading and trailing spaces from entered usernames.
Please note that the spaces are allowed in the local part of email addresses (the part before @) but must be quoted and therefore can’t be leading. Spaces are not allowed in the host part (the part after @) at all. Therefore, per https://en.wikipedia.org/wiki/Email_address#Invalid_email_addresses, any address with a leading or trailing space is invalid.