TypeError: descriptor is undefined

I’m adapting a custom field plugin to Jira 10.3.3. After changing from spring scanner to spring configuration, nearly all is working, except the searcher velocity template rendering.

As soon as I add the field to the (basic) search, JavaScript console reports an error in SearcherGroupListDialogView.js:

Uncaught TypeError: descriptor is undefined
    _searcherSelected http://localhost:2990/jira/s/480f003c01561d6272c3230611dbeedc-CDN/8lllcb/10030003/1bqcfwe/20.1.15/_/download/resources/com.atlassian.jira.jira-issue-nav-components:query/SearcherGroupListDialogView.js?locale=en:105
    jQuery 2
    selectItem http://localhost:2990/jira/s/d84ed91085d9ae73648c813a8aee6cf2-CDN/8lllcb/10030003/1bqcfwe/1.0/_/download/resources/jira.webresources:frother-checkbox-multiselect/CheckboxMultiSelect.js?locale=en:306
SearcherGroupListDialogView.js:105:64

Searcher definition:

    <customfield-searcher name="FixBrancher Multi Select Searcher" i18n-name-key="customfield.fixbranches.searcher.name"
                          key="fixbranches-searcher" class="com.besi.jira.plugins.customfield.searcher.FixBranchesSearcher">
        <description key="admin.customfield.searcher.multiselectsearcher.desc">Search for multiple values using a single select list.</description>

        <resource type="velocity" name="search" location="/templates/customfields/fix-branches-custom-field/search/search-multiselect-edit.vm"/>
        <resource type="velocity" name="view" location="/templates/customfields/fix-branches-custom-field/search/search-multiselect-view.vm"/>
        <resource type="velocity" name="label" location="/templates/plugins/fields/view-searcher/label-searcher-basictext.vm"/>
        <valid-customfield-type package="${project.groupId}.${project.artifactId}" key="fix-branches-custom-field"/>
    </customfield-searcher>

In the log I found some warnings (I’m not sure whether they have something to do with it):

2025-03-11 11:59:17,842+0100 http-nio-2990-exec-5 url: /jira/browse/BIB-8, /jira/secure/IssueNavAction!default.jspa; user: admin WARN admin 719x5595x1 psvv3u 127.0.0.1 /browse/BIB-8 [velocity] Cannot locate method(s) getDescriptor(), getKey() for class com.atlassian.jira.issue.customfields.impl.ProjectCFType for security introspection in Velocity

Maybe anybody has a hint how I could further analyse that? Any help is appreciated, thanks a lot!