You don’t provide info about your operating system and setup, etc. as required (see Psychtoolbox-3 - Psychtoolbox Forum), so hard to judge, but:
The sync failure is unrelated. If it happens frequently then is points to a buggy or misconfigured display driver, iow. your visual stimulation timing is broken. Happen a lot on MS-Windows with Intel graphics, and on various broken macOS operating systems. help SyncTrouble.
The specific error message you observe usually happens if no suitable font can be found, due to some problems with the libfontconfig library or its fontconfig database. Happens especially often on some macOS machines due to Apples shoddy operating system, less often on MS-Windows, essentially never on Linux. Cfe. External text renderer plugin failed to render the text string (Windows)
Make sure you use the latest PTB Beta, as it contains the latest workarounds for operating system bugs. If you’d need further assistance from myself, help PsychPaidSupportAndServices.
-mario