Can this error occur due to mixing VHDL and Verilog files in the same project?

Yes, this error can occur in mixed-language projects. While most modern FPGA tools support VHDL and Verilog co-existence, specific configurations are often required: 1. **Language Configuration:** The project must be correctly configured to handle both VHDL and Verilog source files. 2. **Instantiation Syntax:** The syntax for instantiating a module from one language into another must be correct (e.g., VHDL's `component` declaration and `port map` for Verilog modules, or Verilog's direct instantiation with `.` for VHDL entities). If 'dff' is, for instance, a Verilog module being instantiated in VHDL without proper component declaration or mixed-language settings, the VHDL compiler might report it as undefined.