The camera has two limitations built into it, as I understand it. One is the limit of 29'59" which was imposed by the EU to differentiate true video cameras from DSLRs that do video. The former attract a much higher duty rate, and Canon adopted that limit to avoid the tax. Remember a DSLR is a stills camera that can do some video.
The other limitation is a 4 GB video max file size. Both of these are described in your manual on P167. There is no way to get around those, and you are most likely hitting the individual file limit. If all you want to do is use a webam and not record, then I would suggest get a dedicated webcam - they are built for the job. If you want to record for longer periods then you need to look at other camera models.
cheers, TREVOR
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