It seems that a recent update of Bitdefender Endpoint Security is blocking the 64 bit version of PL/SQL Developer. The solution is to add the PL/SQL Developer installation directory to the exception list in Bitdefender. For example:

C:\Program Files\PLSQL Developer 14
C:\Program Files\PLSQL Developer 13
C:\Program Files\PLSQL Developer 12

We are investigating the issue and will post any updates here...