Trouble with undefined Object in VBA Run-Time error '91' - vba

Question

I've been working all week to prepare a VBA application, which I'll be using in a meeting today. Unfortunately the code that has been running all week last week without a hitch, has decided to break over the weekend.
I constantly get Object variable or With block variable not set Run-time error '91' from this statement:
With Sheet5
Set adjrng = .Range(.Cells(.Range("G43:G60").Find(.Range("H39").Value).Row, 10), .Cells(.Range("G43:G60").Find(.Range("H39").Value).Row, 21))
End With
idea is to set a range in the row of the Range G43:G60 where the Value of H39 matches from Column 10 to Column 21.
Anybody spot the issue? My brainz are to nervous and sleepy this morning...
Thanks a bunch
Ben
EDIT:
After playing a bit with find and replace, the issue seems to be that excel has not yet properly calculated the "lookin" and "lookup" Ranges G43:G60 and H39. A simple recalculation didn't make excel rediscover the contents but when I used one of my input toggles to display a different value in those cells, and the went back to the original it did manage to find it.
Maybe using find for this is bad style, the find formula has these kind of hicups usually or any other comments on this? For now everything works fine again, but I'm afraid of running into these issues again. Any tips would thus still be appreciated.
EDIT: (from comment below)
We have a dynamic range (G43:J60) where unique identifiers are listed in column G and data is to the right. if something is changed in the data part of the range AND the lines uniqued identifier in column G matches the one in cell H39 a sub() is triggered via worksheet_ change intersect(target, adjrng) Defining that adjrng is the part that throws errors when find returns null.

Answer 1

I believe you are simply trying to set a range hoping that there will be two matches to the value in H39 within the G43:G60 range. While I avoid on Error Resume Next (never could adjust to the logic of breaking something in the hope to accomplish something), I always check that the values will be there when I look for them.
Dim rwUNIQ as long
Set adjrng = nothing
With Sheet5
if cbool(application.countif(.Range("G43:G60"), .Range("H39").Value)) then
rwUNIQ = application.match(.Range("H39").Value, .Range("G43:G60"), 0)
Set adjrng = .Cells(42 + rwUNIQ, 10).resize(1, 11)
end if
if not adjrng is nothing then
'do something with adjrng
end if
Set adjrng = nothing
End With
That checks to make sure that there are at least two H39 values in G43:G60 before proceeding. There is no further error control because we've counted at least two of them. You might want to compensate with an Else for when there isn't. If a single H39 value was found, you also might want to select a single row.
Remember that the .Find uses many parameters that were retained from the last time Find was used, whether in VBA or with a user on the worksheet. You have a real lack of parameters that specify the options that Find should use to proceed. e.g. xlPart or xlWhole, After:=what?, look in formulas or values, etc.
EDIT:* Modified the code to look for a single instance of the value in H39 and .Resize to expand the width (as per OP's comments).