This COULD be a bug that I just addressed.
Does your clip from SOURCE project contain BUILT-IN PLC elements that go BEYOND the DEFAULT range (or even beyond the range in your DESTINATION project). i.e. Your SOURCE clip references the built-in timer structure T999 (e.g. TMR T999), but in your Destination project, the built-in T only goes up to T255 (i.e. 256 timers)?
You will get this error in THAT situation.
To resolve this with the current release, you must either
1. IN THE DESTINATION PROJECT - bump up the size of your built-in data block to be the size of the data block in your SOURCE CLIP, or at least the size of the largest element that is in the clip (1000 in my example, because T999 is the 1000th timer).
or
2. IN THE SOURCE PROJECT - replace the element with one that fits within the range in the destination project. REMEMBER TO NOT SAVE THE CHANGE TO DISK IN THE SOURCE PROJECT!!