Can’t update Ultimate Member, error messages/no access to WP functions.

1. I have seen the following message since one year, they recommend I save my website ?before doing the upgrade of Ultimate member but I don’t know how to do it and if this app concerns me or not, could you please explain what I need to do ?

Ultimate Member version 2.7.0?needs to be updated to work correctly.
It is necessary to update the structure of the database and options that are associated with?Ultimate Member 2.7.0.
Please visit?“Upgrade”?page and run the upgrade process.

2) I can’t login anymore with my admin. email address, only with my ID why ? I get erro msg : “Value error thrown, missing format specifier at end of string”. But it works if I click the link at the bottom, on the name of my Website, why ? i even tried my 2 email adresses, one for admin., the other for my WP account.

3) –??I get also the above mentionned error msg as well as when I click on Plugins in the Menu on the left vertical bar of WP, how to fix it, why ?

thanks, best regards.
Béatrice?

The page I need help with: [log in to see the link]