Performing "SUMIF" where "range" & criteria" are texts

I got that formula: =SUMIF('[DESPESAS COM MATERIAIS.xlsx]DESPESAS COM MATERIAIS'!$B$4:$B$50,'TRABALHOS CONTRATADOS.xlsx'!$F$5,'[DESPESAS COM MATERIAIS.xlsx]DESPESAS COM MATERIAIS'!$H$4:$H$50) wich "range" & "criteria" are texts. The problem is the formula works well. We close the excel, go do something else and when we came back to open the excel to change some datas on those sheets, sometimes we can see some errors like "VALUE" or "REF" in some sheets. Everything is fine, but those errors still coming back. Than we correct that again, go do another thing. When we open excel again, there they are again in another sheet.
So we keep fixing those errors and everything looks fine. They're not supose be there.
Any help what we're doing wrong? The Math formula doesn't work with "texts" or what ?
Thank you guys for any help.