Login / Login failure - IE, Chrome, Firefox

Since today I have a big error. Without changing any plugins, code, etc. we have the following behaviour on one of our sites (events+, membership pro 2 installed)
Somestimes we can login - sometimes we receive the error wrong user name or password.
Especially with IE and the membership pro login feature we have in addition the behavior that after clicking log-in we receive the message please wait - and nothing happens.
Has anyone else had these kind of errors. Security plugins like wordfence, etc. are installed and are showing that the site is clean.
Many thanks in advance for any advice.
Kind regards,