Since you are on SQL2008, you can forget about TRY_CAST since this is a SQL2012 feature only.

On SQL2008 you could use a CASE expression to guard the expression evaluation to avoid this issue.

Like this:

UPDATE#t2

SETTTT_Factureren = 0,

TTT_FactuurStatus = 0,

TTT_FactuurNummer = 0

WHERETTT_ProjectID = 26

AND TTT_FactuurStatus = 1

AND TTT_ID NOT IN(

SELECT

CASE WHEN X.TextDescription LIKE 'Int.nr.TXT:%'

THEN CAST(SUBSTRING(CAST(X.TextDescription AS VARCHAR(100)),13,10) as int)

ELSE -1

END

FROM #t1 X

WHERE X.TextDescription LIKE 'Int.nr.TXT:%'

)

Note the use of LIKE which gives much better performance than SUBSTRING in the predicate

This method is also more efficient than casting TTT_ID to a varchar since it is much faster to compare integers than strings.

Good luck