I think I'd try a password manager to test this. I favor Last Pass, but there are several. Set it up on your computer first and enter one or two of the problematic logins, then make sure the manager enters them correctly and that you can log in. Now install that manager's companion app to your tablet and see if you can log in.
This will eliminate any question that the login info is being entered correctly on your tablet. If you can login in this way, then I'd re-examine your manual entries. If possible, use a site's "show password" option to verify that's being entered correctly. If that option isn't available, it should at least display the last character you entered for a second for you to double check.
If using the password manager fails to log you in, then we'll know this is something related to the tablet or perhaps the websites you're visiting.