Renaming Virtual Servers by loading configuration in v11.5.1 is bugged. It does not work as previously, and any of the public hotfixes have not yet addressed the problem.
You can circumvent the error if you rename Virtual Servers that have no Default Pool attched to them in Virtual Server settings. Obviously, removing the link between LTM Pool and Virtual Server is not an option as it comes with an impact. To successfully rename a Virtual Server without an impact, you will have to rename your Virtual Servers on a standby appliance, and then sync to active appliance only after you have restored the Default Pool configuration in Virtual Server settings.
1) Remove Default Pool in VS config
2) Rename Virtual Server in bigip.conf
3) Load configuration
4) Restore Default Pool config in Virtual Server settings
5) Sync from Standby to Active
Just upgrade to v11.5.3 HF2. Save yourself from having to deal with this crap every time.