I suspect that the cause of the problem is that you have a row in the data where TextDescription begins with 'Int.nr.TXT:' but there is no valid integer after.

This is a perfect time to use one of the new functions in SQL 2012: TRY_CAST()

TRY_CAST will return null if there was a problem with the cast instead of generating an error.

Your code could then be rewritten as:

UPDATEtblInvoicesProject

SETTTT_Factureren = 0,

TTT_FactuurStatus = 0,

TTT_FactuurNummer = 0

WHERETTT_ProjectID = 26

AND TTT_FactuurStatus = 1

AND TTT_ID NOT IN(

SELECT isnull(TRY_CAST(SUBSTRING(CAST(X.TextDescription AS VARCHAR(100)),13,10) as int), -1)

FROM [500].dbo.frstx X

WHERE (LEFT(CAST(X.TextDescription AS VARCHAR(100)), 11) = 'Int.nr.TXT:')

)

Note the use of isnull to convert any failed casts from null to -1. I assume that -1 is not a valid project number.

If you have a null value in NOT IN () the results will probably not be what you expect.