If you would try to submit simple configuration data with scalar value into single import form it crashes with exception Argument 2 passed to Drupal\config\StorageReplaceDataWrapper::replaceData() must be of the type array, string given.
Check data type in ConfigSingleImportForm::validateForm() and display error message if submitted data is not array.
Closed: duplicate
11.0 🔥
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Based on @alexpott's comment and the fact that there is no work here since then, I'm going to close this as a duplicate of 📌 Validate configuration schema before importing configuration Needs work .
Please feel free to reopen if anyone feels this should stand on its own, however, be aware that @alexpott has advised that special handling for the simple importer is not desired.