Our survey returned an internal 500 server error. It was related to the LimeSurvey platform rather than a single questionnaire.
Based on the available survey configuration the error was the file attachment (File Upload) field.
The survey contained an attachment question where respondents could upload supporting documents. During processing, LimeSurvey attempted to evaluate or validate the field value as if it were a numeric value. However, the attachment field returns text-based data (such as file metadata, filenames, or JSON content), which can cause calculation or validation functions to fail.
The issue became particularly visible when a respondent uploaded the maximum allowed number of attachments. In that scenario, the attachment field returned a larger structured value, and LimeSurvey failed while processing the response, resulting in an HTTP 500 error and a PHP exception similar to:fmod(): Argument #1 ($num1) must be of type float, string given.
As a result, survey submission failed even though the uploaded files themselves were valid.
The attachment field configuration was changed, especially:Maximum number of files allowed.
Reducing the maximum number of attachments or removing the attachment field from calculations and validation expressions should prevent the error from occurring again.
Note: The root-cause section above is based on the investigation and troubleshooting conclusions discussed around the issue. The enterprise search results confirm the existence of the LimeSurvey-related server error but do not contain a detailed technical diagnosis of the attachment-field processing failure.