Hi Bob,
Sorry for the delay, I only just got a chance to publish your survey and take a closer look. Yes you are correct, the string length set for that field is shorter than then length of the string you are trying to submit via that calculation. When I use the field app and submit to AGO I get the following error which explains the error well:
The error message you reported in the original post appears to be different, and I believe this is because you are using Portal and I tested your survey with AGO, and therefore the error message the server is returning is different. Unfortunately the error message displayed is what comes from the server directly and may differ depending on Portal version and configuration.
Hope this helps and glad you were able to find the problem yourself.
Phil.