Please upgrade to 18.104.22.168 as soon as possible. Especially if you installed PMPro fresh from version 1.7 or 22.214.171.124.
There was a huge bug with the DB generation in version 1.7 and 126.96.36.199 of the plugin, which was causing issues on fresh installs of PMPro 1.7 or 188.8.131.52 (if you upgraded from an earlier version, you would have been okay).
Issues included not being able to change someone’s level, checkout not redirecting, and changing of levels just generally not working across the plugin.
Upgrading to version 184.108.40.206 that was just pushed out should fix your problem. If not, please open a support thread on the WP.org forums or on the site here.
Needless to say, this was a big issue and should have been caught in testing. We are updating our testing methods to catch things like this in the future (i.e. doing more testing on fresh installs of the plugin and specific upgrade paths).
Thanks and sorry for the trouble this has caused.
p.s. For those interested, the issue arose as we have moved from using simple DB queries to setup the PMPro DB tables to using the WordPress dbDelta() function. During this switch we updated the CREATE TABLE queries in the plugin to reflect the latest database schema for the PMPro tables. Somehow we missed the id and status fields on the pmpro_memberships_users table. These fields were added in version 1.5 (they allow us to better track level activations and cancellations for reporting and are also part of our effort to allow multiple membership levels per user). Version 220.127.116.11 of the plugin checks for these columns in the pmpro_memberships_users table and if not found will re run the upgrade scripts from the 1.5 upgrade. Furthermore, in all of our test sites, we either started with an older version of PMPro and upgraded to the latest version or installed PMPro with some of the old tables still in place and generally dropped the ball on testing PMPro fully on brand new fresh installs.