Alex Earl I see with below commit  you removed the code that Leo Hu mentions, what was the reason for removing, Leo's solution not work?

I still get this error when using this plugin in a Jenkins pipeline

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

Awesome I figured it out!
Modified the code to use the resURL instead of the rootURL
rootURL - the Jenkins instance
resURL - static webapp resources such as JavaScript or HTML (cf. Jenkins.RESOURCE_PATH)

Not sure why resURL works while rootURL doesn't but it might be that rootURL doesn't have a proper context path in our setup or something so it might not be pointing to localhost:8080.

Changed checkUrl below, rebuild plugin in maven, installed manually and it worked.

checkUrl="'${it.resURL}/descriptor/hudson.plugins.validating_string_parameter.ValidatingStringParameterDefinition/validate?regex='+encodeURIComponent("${it.jsEncodedRegex}")+'&failedValidationMessage='+encodeURIComponent("${it.failedValidationMessage}")+'&value='+encodeURIComponent(this.value)"

I'm still having the same problem now after the latest jenkins update.
I was able to pinpoint what was causing the "ERROR" message, it's the index.jelly file under

src/main/resources/hudson/plugins/validating_string_parameter/ValidatingStringParameterDefinition/index.jelly

<div name="parameter" description="${it.formattedDescription}">
                        <input type="hidden" name="name" value="${it.name}" />
                        <f:textbox name="value" value="${it.defaultValue}"
                                *checkUrl="'${it.rootUrl}/descriptor/hudson.plugins.validating_string_parameter.ValidatingStringParameterDefinition/validate?regex='+encodeURIComponent(&quot;${it.jsEncodedRegex}&quot;)+'&amp;failedValidationMessage='+encodeURIComponent(&quot;${it.failedValidationMessage}&quot;)+'&amp;value='+encodeURIComponent(this.value)"*/>
                </div>

There's something wrong with the checkUrl descriptor that errors out..wish it was easier to read so I can debug it. Removing it removes the ERROR, but probably removes the regex validating.

If anyone knows how to fix it, please let me know.

I had a similar issue, but when I installed the plugin and tried to use it, it would always show the text "ERROR" below it but I think the regex match work.
This made the plugin unusable since people wouldn't know when/if they were typing in an error.
However, I was able to fix it by doing a fresh install of jenkins through a build after downloading version 2.3 github repo.(NOTE version 2.4 doesn't work)

wget https://github.com/jenkinsci/validating-string-parameter-plugin/archive/validating-string-parameter-2.3.tar.gz
#install maven
mvn -X package
#Uninstall the validating-string-parameter plugin + deleted the directory /var/lib/jenkins/plugins/validating-string-parameter
#Manually uploaded the 
validating-string-parameter.hpi 

Pretty crazy workaround since I literally used the same code as in the plugin downloaded from jenkins but it seems the custom build and upload worked for some reason. I didn't see anything different in the jelly conf or java files so it's strange but this works. Hope this helps someone out since this parameter plugin is pretty essential.