Jenkins login issue

As stated in this answer, the quickest way to get around this would be to disable security altogether by making the following change to your Jenkins config.xml file:

<useSecurity>true</useSecurity>

After rebooting your Jenkins instance, you should be able to return to the admin page and set everything up from there.

An alternate solution to this is mentioned here, which should accomplish what you want whereby preserving the security of your Jenkins instance (thus making this a more ideal solution). You would instead update the passwordHash section of your Jenkins config.xml file:

<passwordHash>#jbcrypt:$2a$10$razd3L1aXndFfBNHO95aj.IVrFydsxkcQCcLmujmFQzll3hcUrY7S</passwordHash>

The passwordHash section of the config.xml file is a child tag under <hudson.security.HudsonPrivateSecurityRealm_-Details>.

You would again need to reboot Jenkins once this change has been made, and you would login using the password 'test'. Of course, you could provide your own hash of a different password which would be advisable.