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.
SET TTT_Factureren = 0,
TTT_FactuurStatus = 0,
TTT_FactuurNummer = 0
WHERE TTT_ProjectID = 26
AND TTT_FactuurStatus = 1
AND TTT_ID NOT IN(
CASE WHEN X.TextDescription LIKE 'Int.nr.TXT:%'
THEN CAST(SUBSTRING(CAST(X.TextDescription AS VARCHAR(100)),13,10) as int)
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.