Personal Tax/VAT- Invocation of the constructor ‘CefSharp WPF Chromium Web browser’ etc.

personal-tax-vat-invocation-of-the-constructor-cefsharp-wpf-chromium-web-browser-etc

Personal Tax/VAT- Invocation of the constructor ‘CefSharp WPF Chromium Web browser’ etc.

28th May 2026

You get this error when using PT MTD, PT Data retrieval and the VAT filer. As they use all a online LINK to HMRC and requires Chrome to be run. There have been reports of this happening at random times but working fine a moment /later on.

If you are Not using HMRC data retrieval/MTD when working on PT for this affected client, then you can ignore the warning by clicking ‘OK’ (like running a tax comp etc) and continue work.

1. If it affects one/specific clients only, then click ‘OK’ on the warning and see if it allows to proceed, if not then update IRIS to latest version, do restart your PC and IRIS and VAT , and if nothing works then reset your login in the ‘Authorised Agent screen’ and log in again (Note: Do not use the ‘Agent Services Sign up’ button). If it continues then read the steps below.
2. If it affects ALL clients and clicking ‘OK’ doesn’t work and you cannot do work in PT/VAT and neither a restart of PC/Server works then read the steps below

There is a possible local IT issue on your PC/SERVER/HOST SITE which is blocking a HMRC login screen from showing on your Browser (It blocks the chromium process to start) and you get error: Without this HMRC login screen – you cannot sign in and get the PT/VAT from working.

a) VAT obligations will not appear and cannot submit

b) PT data retrieval will not work for a client

c) PT MTD Quarters do not appear and cannot submit

As this could be environment related on your pc/server/host, so ask your IT team to review these areas.

1. Go to IRIS, Setup- Authorised Agents – Do you have a row with the relevant IRIS tax product next to your name? (Pre-pop/Self assess/VAT) if its missing then the HMRC login may not appear and thus cannot sign in due to the error and continue with the fixes below. IF the row appears with your name- check the date next to it and restart your PC and try again. If you continue to get the error and OK doesn’t work then contact IRIS Tax support (as it may need a reset).
2. Review all your anti-virus and local security setups especially if you have installed a 3rd party software or using a Server/Hosted site with them installed. Each of this programs/apps etc have different designs to exclude/include/block programs and sites. This can block IRIS and Chromium etc.
3. If you have an antivirus, anti-malware etc installed – you can test this by switching them off and then try and use PT/VAT. If it then works you may have found the cause.
4. Check your IT setup is allowing Chrome and Edge to be run. This is because it requires the browsers edge/chrome to be setup correctly/fixed or the connection with HMRC will fail and you get that error.
5. A user reported back to us they had to get Edge/Chrome ‘to be added to exclusion’ which then stopped the warnings and allowing VAT/PT users to work again.
6. Check if you have software which is already using the chromium subprocess – a user had ‘CCH’ running which caused this issue
7. Change the running of the application in the ‘admin mode’
8. Check if you have these CEF/other files under the ‘IRIS’ Folder on the affected pc/server etc – any missing may be the possible cause