Hello regner,
Please send the problematic .SRT file to support@agisoft.com or share the download link for it, if there's no sensible information, so that we could analyse the format specifics.
Recently there were several reports related to some variations of SRT format, based on that feedback we have improved the compatibility of Import Video feature, but looking at your file will help us to understand, if the planned fixes are already sufficient, or whether we need to implement the support for another variation.