Civicrm stuck when renewing membership
It reminds me of a similar issue I had at a client last week: we received a 403 error when we tried to edit memberships.

The error was due to an anti sql-injection mechanism that was in place at the hosting provider. It could be turned off via the control panel.

Can you check if something similar is active in your installation? Either in the control panel or via a Wordpress security plugin? Ultimately, ask the hosting provider.