When compiling packages in Delphi (as the JCL and JVCL do) that require other packages at compiletime the compiler uses the dcp files for the required packages and not the bpl files of those packages.
For example the JvCore.dpk
has Jcl
and JclVcl
in its required clause among some RTL packages. That means when compiling it will take Jcl.dcp
and JclVcl.dcp
to get the information where the procedures/classes it needs are located in the bpl.
These dcp files are most of the time (unless specified differently - see next paragraph) located in the DCP folder which is right next to the BPL folder.
If the dcp it uses is not compatible to the bpl it finds when loading you will get those errors. Make sure you don't have any old bpl and dcp files anywhere that the compiler may find. In the JCL setup you can specify where it puts those while the JVCL setup puts them into the same directory as the dcu files.
Additionally if you have different Delphi versions installed it can get worse because usually the dcp files don't have a suffix. If the directory it puts those happens to be in the PATH variable wrong versions might be found. However the DCP directory is not put into PATH by the Delphi installation but only the BPL directory.