Thanks for the follow up.
Upon closer inspection of the original problem, there's no need to drop anything. dBpoweramp attempted to first write the file with a single 'padding' chunk large enough to fit all tags, allowing tags to be written in its place after encoding. This would cause libFLAC to fail with the strange error messages you encountered.
This has been revised for the next update (link below), though beta 4 is safe to use, there are no bad behaviors or dropped tags (disregard the info lines about dropped tags, nothing is lost).
As for "error opening file", there seems to be some recurring glitch with reading from network shares - I don't think file content is relevant, as dBpoweramp is unable to open the file, doesn't get as far as reading its content.
I added additional debug logging in today's beta-
https://www.dbpoweramp.com/beta/DMC-R173b5.dmg
If you get it to fail again, please post what it says - there will be extra error codes reported with "error opening file" lines.