First, check if the control is registered properly. To do this, create new form and try to put this control on it. If you do not see the control in the list of controls, probably control isn't registered. From Start menu, run this command:

regsvr32 {full path to the control}\{name of the control with OCX extension}

for example:

regsvr32 C:\WinNT\System32\SampleControl.ocx

If you register properly control and see it in control list, but cannot add the control from ToolBox to the new form, probably control requires license and you do not have it.

If control is properly registered and you can put them on new form, but error still appears with your old form, the reason can be in changing of control interface in new version. If you can, send me the control (.OCX file) and the form and I'll try to solve the problem.