Hello there, Dale45.
I appreciate the detailed information you provided about the error message you received while changing your customer's details in QuickBooks Self-Employed (QBSE). To resolve this issue, we will go through some basic troubleshooting steps.
Start by refreshing your browser to fix any glitches and ensure the page loads correctly. Alternatively, you can use a private or incognito window to address temporary issues. After doing so, attempt to update your customer's information again.
If the problem persists, consider switching to a supported and updated browser to eliminate any browser-specific issues.
Please feel free to revisit this thread with any updates or further questions.