The following pipeline shows an ERROR link instead of showing the configured error message.

stage('Input stage') {
    def res = input(
        parameters: [[
            $class: 'ValidatingStringParameterDefinition',
            name: 'param1',
            failedValidationMessage: 'Input must be a number', 
            regex: '[0-9]+'
        ]]
    )

    println res
}

Works with version 2.3 of the plugin. Reproducible with Jenkins 2.89.4 and 2.138.1.

Commit: https://github.com/jenkinsci/validating-string-parameter-plugin/commit/9caac7b713976af9363cd944f89cadd44fbb4652#diff-12cb1c400787ab164b54d0ef8d0cbda9

The problem seems to be the relative URL. The base path for the input form is /job/<JOB_NAME>/<BUILD_NUMBER>/input/ so the href descriptorByName/hudson.plugins.validating_string_parameter.ValidatingStringParameterDefinition/validate is not available.