Do you have a large number of UDFs in SO_SalesOrderHeader?   We had the same issue in a test upgrade from v4.4 to v4.5.

The answer from Sage was that the customer had too many Sales Order header UDFs. In 4.5 a vanilla system is attempting to handle 976 fields and if the total number of fields goes past 999 then the error occurs. The suggested workaround is to reduce the number of UDFs because MAS can't sequence more than 999 fields. 

I'm told this is fixed in v5.0 (2013).

I have a total of 14 UDF's in the SO_SalesOrderHeader, with 145 fields used, and 1960 of 20,000 bytes.

So if I understand this right, 976 fields is what MAS can handle without SO_SalesOrderHeader without any UDF's? With UDF that's 999?

I'm not sure I understand your question. 

Without any UDFs the SO_SalesOrderHeader lookup loads up 976 fields.  If you have 24 or more UDFs in SalesOrderHeader or other related tables (AR_Customer, AR_CustomerCreditCard, AR_SalesPerson, SO_ShipToAddress) or UDFs in related UDTs, then the handler counts past 999 and you'll get this error.

Sage knows about this issue.  If your cause is what I outlined above, I'd suggest contacting technical support.

Here's a screenshot on my UDF status for SO_SalesOrderHeader. Am I exceeding the limit?

Not with this table.  Do you have 10+ additional UDFs in AR_Customer, AR_CustomerCreditCard, AR_SalesPerson, and SO_ShipToAddress?

I don't have 10+ additional in those tables. My UDF count for:

AR_Customer - 3

AR_CustomerCreditCard - 0

AR_SalesPerson - 0

SO_ShipToAddress - 0

I do have a few UDFs in other tables though, not sure if they would affect the S/O header:

SO_SalesOrderDetail - 14

SO_InvoiceHeader - 6

CI_Item - 22

SO_InvoiceTracking - 24

AR_InvoiceHistoryTracking - 24

RA_ReturnHeader - 2

My list of tables to check against UDFs was short (I was looking at v4.4).  Check out each table (including UDTs) in the Lookup Customization Wizard and count the UDFs. If you're 24+ then this is probably the issue.  If not, interesting coincidence.

Hi Jerry, thanks for all the info though. I just looked up in the customization wizard, these are what's being listed there. I don't think I have 24+ UDF's in these tables. Maybe it is something else?? I'm scratching my head badly now

AR_CUSTOMER
AR_CUSTOMER_CREDITCARD
AR_DIVISION
AR_PAYMENTTYPE
AR_SALESPERSON
CU_COMPANY
CU_OPPORTUNITY
CU_PERSON
CU_USERS
IM_WAREHOUSE
JC_JOB
SO_CANCELREASONCODE
SO_SHIPPINGRATEHEADER
SO_SHIPTOADDRESS
SY_COUNTRY
SY_SALESTAXSCHEDULE
SY_USER
SY_USERHISTORY
SY_ZIPCODE

Update: I was able to fix half of the problem - the lookup customization now wouldn't crash on SO Invoice Data Entry.

I removed 1 UDF from AR_Customer (now has 2), and removed 2 UDF's from SO_SalesOrderHeader, and now the lookup list can be freely customized in SO Invoice Data Entry.

The problem still remains for SO_SalesOrderHeader. The lookup list woud crash when I tried to customize it.

I duplicated my UDF's from production server to my test server. My tes server would load up fine, but the production server would just crash.

I also double checked the tables that the lookup list loads for SO Data Entry, and compared to the tables that the S/O Invoice Entry loads. Found only these 4 tables that the SO Data Entry loads, but SO Invoice Entry doesn't load:

SO_CANCELREASONCODE
SO_SHIPPINGRATEHEADER
SY_USER
SY_USERHISTORY

I do not have any UDF's in these tables. Here's the screen shot of the crash. Maybe the problem is in SY_USER or SY_USERHISTORY?