I'm thinking the String is probably too long. The max length of a String in Access is, I believe, 255 characters.

How are you storing/creating Inv-T2? Could be an issue with the 255 character limit depending on how you're doing it.

Inv-T2 is extracted from a long text field ands it is populated with xml code. This could hold hundreds of values (meat catchweights) Example "<CatchWeights><CatchWeight>10</CatchWeight><CatchWeight>10</CatchWeight></CatchWeights>"

I need to find a way to sum that field.

It's a long text field in a table? You could just parse the data and sum it with vba. That's more advanced though.

Is it always this syntax?
Number(space)(plus)number

Or could there be no spaces or subtraction?

Sorry, I didn't see the space.  Fixed now example: 0+10.00+10.00+0

vba would be the best, my understanding is I need to convert it to a list then sum the list.  I'm not skilled enough for that :(

Does it always start with 0? Can it be - instead?

Assuming it can only be +. Something like this should work. It assumes you are storing the string value in a table and setting the sum value in a table. This code is in a button on a form and when pressed sums the results in the table.

There may also be more involved if your query is doing more to get the number to that point.

Edit: I made it Short Text in my picture, it works fine with Long Text too and you'd need to use Long Text for the String Value if it was over 255 characters.

The sum is actually going on a report, query supplies data for the invoice report.  instead of a button can it be a field on report?

In my example you're going to need to use a table. You may also be able to make a module to run the calculation inside a query.

Edit: This is doing it in a module/query in the image below.

I must have something wrong, it give me a result of 0

Typo: you typed "Sun" instead of "Sum" in the code.

To confirm, you have MyStringValue stored in the table as Long Text right?

Is there anything else going on in the field properties. Any formatting or input masks or anything?
Are there any spaces or multi-line in your values?

Something else is off. Mine works great. If everything looks normal and it doesn't work, then you're probably going to want to try it in a new database with bare minimum fields/setup like my example photo, and add things on until it breaks and we find the problem.

Or that would do it. Nice eye Kevin, I didn't even notice that.

The real culprit is the lack of Option Explicit, which would've caught the typo as an unknown name.

When I rerecord Developer 1, I'm going to put Option Explicit at the top of the class.