We are on the Portal running 10.5.1 and pushing up services that have been working. Then all of a sudden, yesterday, we cannot push any of our surveys up. The first time we publish, "ERROR: Creating feature class failed. Error 500" The second time, "ERROR: 400." This occurs even with a three question survey.
It sounds like there may be an issue with the Data Store component of your ArcGIS Enterprise deployment. You may want to review the Data Store's logs - Access and manage ArcGIS Data Store logs—Portal for ArcGIS (10.5.x) | ArcGIS Enterprise; additional help with troubleshooting is at Troubleshoot ArcGIS Data Store—Portal for ArcGIS (10.5.x) | ArcGIS Enterprise.
Along with what James recommended, are you able to create new hosted feature layers from within Portal? If so, does your federation validate successfully from Portal?
If federation validation fails, try to validate the ArcGIS DataStore connection from Server Manager. If it does not validate here, check the Server logs for extended information.
That Esri technical support article discusses the possible reasons of:
The data store is automatically set to read-only mode when the drive on which the data store is hosted has less than 1024 MB of available disk space.
a known limitation on the size of the table supported on the portal.
workaround is to run these command line utilities:
changedatastoremode utilityupdatebackupschedule utility