Here's the scenario:
TDA50 with DHLC4, DLC4 and two (2) LCOT4 cards. All cards and ports ENABLED.
Original number of lines from LEC was 6, thereby leaving 2 ports open for 2 addtional CO's.
Orignial number of KX-T7633 telephones extensions was 7, leaving one port open for 1 additional telephone.
THE CHANGE:
Added 2 additional CO lines from LEC using the last 2 ports in the LCOT4 card.
Added 1 new KX-T7633 phone using the last port in the DLC4 Card.
RESULTS:
All of the original 7 KX-T7633 phones now have 8 working CO lines.
The new KX-T7633 added only has CO's 1-4 working. CO's 5-8 are not working.
I attempt to reprogram the "Flexible Buttons" as type "Single CO" for CO5-CO8
When I press "Apply" the error "E000404 Invalid extension number was entered. (Extension block error)" comes up.
Any ideas as to why I cannot program CO5-CO8 to the 8th extension (ext 108) on the LCOT4 card when all 8 CO's are programmed on extensions 101-107?
Thanks...

Any chance it's somehow blocked in Class Of Service settings?

Are any of the other buttons programmed as a DSS key that has a conflict?
Try this, go into flexible keys, pull up ext 101 and use the copy buttons to ext 108.

Thanks guys. @Toner, checked COS. Only 2 classes in effect. Standard COS64 for 101 and COS1 for all others.
@C5Z, I'm trying to post a pic of the "Flexibel Button" graphic here to show that there are no conflicting DSS buttons assigned. No matter what I do, when i open Extension 108, I can only exit by pressing "Cancel". All attempts to "Apply" the new programming cause a E000404 error.

Are buttons 14-36 set to "Not Stored" for ext 108?

I'm going to take a wild shot here and hope that I hit something. Are you sure that this port is extension 108? It could have been changed to something else. While unlikely, you may be making programming changes to a non-existent extension number, while the one you're working with is something unexpected, like 208. The system will let you program an unused port, but you may think you're changing the COS, etc. for the phone on this port, but you aren't. Again, I'm just throwing this out there. Sorry for the late response.

If you figure it out, or more importantly, if the information we've given you here helped you resolve it, please let us know by posting your results. We aren't looking for accolades, but a web search by someone else in the future with the same problem might land them here on this thread. Thanks in advance.

Drum roll please....
...and the Golden Phone goes to...wait for it...C5Z!

I have no idea why simply copying the flex button programming from ext101 to ext108 worked. Sure enough, all 8 CO's are now programmed and working. However, I still have not tried to save any programming changes directly to ext108. Don't want to press my luck.

Thanks again to all. There's no way that I would not have posted anything that was helpful or even not helpful - any CORRECT information is helpful even if it does not fix the problem. The type of assistance provided here can't be found in a manual or a FAQ. We used to help one another much more years ago. I guess I'll have to help some of my old buddies bait their hooks when I'm fortunate enough to join them off the grid.

I did run into one more issue with the TDA50. I'm starting a new thread, so if you are as good as C5Z, your assistance is greatly appreciated.