Intermittent JATOS data logging failure in OSWeb v2.2.9 — result file shows only [

jeboydirk

Platform: OSWeb v2.2.9, JATOS (hosted on university server)

Description of the issue:

I am running an online experiment using OSWeb with JATOS as the backend. The experiment runs correctly from start to finish in all cases, but intermittently (roughly 1 in 5 sessions) no trial data is logged at all. When this happens, the JATOS result file contains only a single opening square bracket [ with no data records inside it. The JATOS server is hosted at a university server (jatos1.psy.vu.nl)

What I observe:

• The experiment loads and runs completely normally from the participant's perspective
• All trials are displayed and responses are recorded visually
• At the end of the experiment the screen gets stuck at the data transfer stage
• In the JATOS result viewer the result entry exists but contains only [
• When it works correctly, data is logged incrementally trial by trial as expected

What I have tried:

1. Originally used the built-in OpenSesame logger item — same intermittent failure
2. Replaced the logger with explicit jatos.appendResultData() calls in an inline_javascript item inside the trial loop — same intermittent failure persists
3. The jatos.studyResultId is set correctly at the start of the experiment — so the session initialises properly

Questions:

1. What does the [ in the result file indicate — is this OSWeb opening the result array before any data is written?
2. Is there a known issue with jatos.appendResultData() in OSWeb v2.2.9 when called repeatedly inside a trial loop?
3. Is there a recommended way to verify within OSWeb whether appendResultData() succeeded?
4. Is there a more robust way to log trial-level data in OSWeb that does not rely on appendResultData() inside a loop?

Any help would be greatly appreciated. This is part of a mobile EEG validation study and reliable data logging is critical.