I wrote an app config and omitted the </app_config> at the end. I clicked check syntax, then apply, and nothing happened. It didn't show me the mistake, and it didn't tell Boinc to read the file. I'm running 1.91 beta.
The BOINC client doesn't accept incorrect files, so it probably gave a warning and ignored the file.
Should your syntax checker not have flagged it?
It should, but I probably never checked on it.
Fixed in 1.92