When you first joined us, did you do that on a Mac or on an iDevice? It would have required a name - mine is IWT - and a password.
If you stored both name and Password (PW) in Keychain and if you have Keychain synced across all devices via iCloud, then you should be able to log in from any Device.
Firstly, try logging in on the Device you used for your very first log in. Confirm that works.
Then, on an iPhone or iPad, try logging in via Safari - ie use Safari to get to the Forums, then log in; rather than use the Mobile app. The Mobile app be misbehaving?
Thank you for your prompt reply .
For the time being, keep using that method - it's what I do all the time.
I wonder if you have the correct Mobile app? In the App Store, I used "Search" and inputted "Mac-Forums" (note the - between Mac and Forums). It comes up as M-Forums Mobile with a small square in which is "Mac" with "Forums" underneath it.