对于这种错误，一般只有两种可能，第一是：thymeleaf在前端没有配置，不过说实话，配置这个只是前端写起来有提示，，，，实际上html是可以识别thymeleaf
第二就是前端代码多了很多没用的东西，比如，要去掉才行，具体什么原因我也不清楚
Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.
Sun Aug 16 00:03:52 CST 2020
There was an unexpected error (type=Internal Server Error, status=500).
An error happened during template parsing (template: “class path resource [templates/login.html]”)
org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: “class path resource [templates/login.html]”)
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:241)
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100)
at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:666)
at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1098)
at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1072)
at org.thymeleaf.spring5.view.ThymeleafView.renderFragment(ThymeleafView.java:362)
at org.thymeleaf.spring5.view.ThymeleafView.render(ThymeleafView.java:189)
at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1372)
at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1118)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1057)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943)