The errors the application are likely reporting are C2 errors. A large number of these errors can cause audible artefacts on Audio CDs where as in a data CD, these are correctable using C2 Forward Error Correction. This also explains why Audio CDs can hold up to 800MB of waves on a 700MB data CD.
Try running Nero CD speed and do a surface scan. It will show green dots for blocks without C2 errors, yellow dots for blocks that require C2 Forward Error Correction and Red Blocks (hopefully none) for CRC errors, i.e. could not be corrected. If you only get green dots with the odd yellow dots about 1/5 the way through, then these are the blocks that plextools may be complaining about. I noticed that the cheaper CD-Rs, e.g. those in spindles tend to get C2 errors, where as good quality (more expensive) CD-Rs generally have no C2 errors at all.
One reason that you’re getting (C2) errors around the 18 - 20 minute mark may be the result of your CD-Writer spinning up while changing write zones, i.e. the first zone at 16x and the next at 24x. If you are writing an Audio CD, I would recommend that you use 16x and this way your CD Writer will write at a constant speed from start to finish. I do that even with my 40x liteon.