the error does not arise on the webspace, but already on the raspberry pi.
I still got no further when fixing the error in the report files. The website is displayed absolutely correctly, except for the reports section. Now I have discovered the following error messages through a note in the syslog file.
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: Generate failed with exception '<class 'NameMapper.NotFound'>'
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** Ignoring template /etc/weewx/skins/Belchertown/reports/index.html.tmpl
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** Reason: cannot find 'relative_url'
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** Traceback (most recent call last):
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** File "/usr/share/weewx/weewx/cheetahgenerator.py", line 326, in generate
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** unicode_string = compiled_template.respond()
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** File "_etc_weewx_skins_Belchertown_reports_index_html_tmpl.py", line 188, in respond
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** File "/usr/lib/python2.7/dist-packages/Cheetah/Template.py", line 1707, in _handleCheetahInclude
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** self._CHEETAH__cheetahIncludes[_includeID].respond(trans)
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** File "_etc_weewx_skins_Belchertown_page_header_inc.py", line 93, in respond
May 1 18:20:23 raspberrypi weewx[12969] ERROR weewx.cheetahgenerator: **** NotFound: cannot find 'relative_url'