@DePiep reports that the "Manage TemplateData" tool is not working in either Chrome v69 64-bit or Firefox 62 on Windows 10. Since ?safemode=1 resolves the problem, this may be due to a user script or gadget.
- Clear cache
- Open Template:See_also/doc (read)
- add ?safemode=1 to end of URL
- open for Edit (with me, traditional edit mode, not VE). The appendix ?safemode=1 did not return, so:
- add &safemode=1 to action=edit URL(needs &) and reload, opens:
- https://en.wikipedia.org/w/index.php?title=Template:See_also/doc&action=edit&safemode=1 (edit screen, looks OK)
- Open Manage TemplateData (button)
- Add paramter, paramter "Test"; Apply;
- Done, parameter added  (success).
- Same for Template:Currency/doc (success when using &safemode=1).
- After both successes, tried again without safemode: failed in both pages.
Running with debug=true, webconsole message in Chrome: "Uncaught TypeError: Cannot read property 'accessKey' jquery.js[...]" (in Firefox: "jQuery.Deferred exception: element is undefined [...]").