Hi Sophie - you can start by clicking the linked file 'utag.js:132' and when you do that, you should then see the utag.js (Tealium loader file) in the Sources tab and you should land on line 132 and be able to trace back from there.
Likely there's a stray variable in your profile which isn't getting set consistently. You want to fix this sooner than later because it may be causing data loss.
Feel free to post a link to the affected site if you'd like, and be sure to come back with questions (or the resolution).
This is almost certainly due to an extension attempting to use utag.DB (the debug logging function) before the utag object has been created.
The likely culprit is a pre-loader extension - these run before the utag object is initialised, so attempts to invoke utag.DB return an error stating that the utag object is undefined and as such has no property "DB".
This error can also be caused if you are using the web companion to switch enviroments but are also bundling tags. If this is the case then I would suggest instead of using the web companion to change envriorments to use the Enviroment Switcher Tealium Tool.