What version of ProcessBook are you using and have you upgraded your ProcessBook recently?
The issue is that the VB engine thinks that there is a symbol, named "Value15" but ProcessBook is unable to find it. If VBA cannot instance a control (in this case, the value symbol) it knows about, it refuses to exit design mode. Why the display got into such a state, where the symbols known to VBA and ProcessBook got out of synch, is unclear.
The best solution is to copy all of the symbols and paste them into a new display. Since "Value15" will not be there to copy, the new display will not look for it.
There is also an option to use the /vbasynch switch.
From the release notes of ProcessBook version 2.16:
A new switch has been added to help resolve various situations in which displays become out of synch between PI-ProcessBook and VBA. To repair these displays, start PI-ProcessBook from a command prompt with the /vbasynch switch. For example:
"C:\Program Files\PIPC\ProcBook\ProcBook.exe" /vbasynch
"C:\Program Files\PIPC\ProcBook\ProcBook.exe" MyDisplay.PDI /vbasynch
This switch causes portions of the VBA storages to be destroyed and rebuilt. Once the display has been opened with this switch, you can save it again to permanently fix the file. If the switch is not present on the command line, PI-ProcessBook will behave normally. If you open a file that is out of synch, you will see a message directing you to use this switch. This procedure will fix the majority of displays; however, some displays may require an additional step to repair them completely. See Known Limitations to diagnose and repair these displays.