Troubleshooting Common Errors

Encountering an error? This guide explains the most common issues and how to fix them.

HMRC Errors

Error 3001: Original Return Already Received

What it means: HMRC already has a CT600 for this period - either filed through TinyTax previously or via another method.

How to fix:

• If you need to make changes, file an amended return
• Click the blue panel that appears:
  File Amended Return →
• Or go to Dashboard → Company drawer → Amend CT600

See our Filing an Amended CT600 guide.

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Authentication Failed / Invalid Credentials

What it means: The Government Gateway ID or password is incorrect.

How to fix:

1. Double-check your 12-character Gateway ID (not your email)
2. Verify your password is correct (case-sensitive)
3. Ensure you're using your company's Gateway login, not personal Self Assessment
4. Try logging into HMRC online directly to verify credentials work

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Invalid UTR / Company UTR Not Found

What it means: The 10-digit Unique Taxpayer Reference doesn't match HMRC's records for this company.

How to fix:

1. Check you're entering the company UTR, not your personal UTR
2. Verify all 10 digits are correct
3. Find your UTR on:

- Letters from HMRC about Corporation Tax - Your CT600 from previous years - Your company's HMRC online account

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Box Validation Errors

"Box 160 must not be greater than Box 155"

This means your losses brought forward exceed your trading profit.

How to fix: Reduce the losses brought forward amount - you can only offset losses against profit you actually have. The remainder will carry forward automatically.

"Box 275 must be less than or equal to Box 780"

Current year losses claimed exceed the actual loss amount.

How to fix: This is usually calculated automatically. Try refreshing the calculation by changing any figure and changing it back.

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Companies House Errors

Invalid Authentication Code

What it means: The 6-character auth code doesn't match Companies House records.

How to fix:

1. Each company has its own unique auth code
2. Check you're using the code for this specific company
3. Auth codes are case-sensitive (ABC123 ≠ abc123)
4. If you've lost it, request a new one

See our Companies House Auth Codes guide.

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Accounts Already Filed

What it means: Companies House already has accounts for this period.

How to fix:

• If filed correctly elsewhere, no action needed
• If you need to file amended accounts, contact Companies House directly
• TinyTax cannot file amended accounts (CH doesn't support this via API)

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Form Validation Errors

"Company registration number is required"

What it means: The company number field is empty or invalid.

How to fix:

1. If you selected a company from the dashboard, try:

- Logging out and back in - Clearing your browser cache - Starting the filing again from the dashboard

1. If it persists, contact support

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"Period end date is required"

What it means: No accounting period has been selected.

How to fix:

1. Return to the form
2. Select a period from the Accounting Period dropdown
3. If no periods appear, check the company is properly set up in Companies House

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"Average number of employees is required"

What it means: The employee count field is empty (required for accounts).

How to fix:

1. Enter a number in the "Average employees" field
2. For dormant companies with no employees, enter 0 or 1 (for director)
3. This is required even if the company had no staff

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Technical Errors

Network/Connection Errors

What it means: Network issue between TinyTax and HMRC/Companies House.

How to fix:

1. Wait a few minutes - HMRC systems may be temporarily unavailable
2. Check your internet connection
3. Try refreshing the page and resubmitting
4. Check HMRC service status

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"Session Expired"

What it means: Your login session timed out due to inactivity.

How to fix:

1. Log in again
2. Return to dashboard
3. Click Continue Filing - your draft data is preserved

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Calculation Errors

If calculated values look wrong:

1. Refresh calculations - Change any field slightly, then change it back
2. Check your inputs - Ensure figures are entered correctly
3. Clear browser cache - Sometimes stale data causes issues
4. Try a different browser - Rule out browser-specific issues

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Submission Status Issues

"Submitted but showing as overdue"

After submitting, status updates aren't instant:

1. HMRC processing can take minutes to hours
2. Check the drawer - Click the status icon for real-time updates
3. Refresh the page - Status may have updated
4. If still pending after 24 hours, contact support

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"Pending for too long"

If a submission stays "pending" for more than 24 hours:

1. Check the company drawer for any error messages
2. HMRC may have queried something - check your Government Gateway inbox
3. Contact support with your submission reference

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Quick Fixes to Try

Before contacting support, try these:

1. Refresh the page (Ctrl+R / Cmd+R)
2. Clear browser cache and cookies for tinytax.co.uk
3. Try a different browser (Chrome, Firefox, Safari)
4. Log out and log back in
5. Check HMRC/CH service status - they may be down

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Still Stuck?

If none of these solutions work:

When contacting us, please include:

• The exact error message
• Your company number
• The period end date you're trying to file
• What you've already tried

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Last updated: December 2025