So my initial issue was a bac USB cable, completely weird since I hadn't touched the setup..
Now, after going back and forth with my provider, I was switched to a new server and had to change the path. Their "help" page was incorrect.
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weeutil.ftpupload: Failed uploading /var/www/html/weewx/index.html to server
vmcp17.machighway.com. Reason: '451-Error during read from data connection#012451-Transfer aborted#012451 0.082 seconds (measured here), 194.36 Kbytes per second'
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: ftpgenerator: (2): caught exception '<class 'ftplib.error_temp'>': 451-Error during read from data connection#012451-Transfer aborted#012451 0.082 seconds (measured here), 194.36 Kbytes per second
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** Traceback (most recent call last):
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** File "/usr/share/weewx/weewx/reportengine.py", line 436, in run
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** n = ftp_data.run()
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** File "/usr/share/weewx/weeutil/ftpupload.py", line 210, in run
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** ftp_server.storbinary(stor_cmd, fd)
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** File "/usr/lib/python3.7/ftplib.py", line 515, in storbinary
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** return self.voidresp()
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** File "/usr/lib/python3.7/ftplib.py", line 251, in voidresp
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** resp = self.getresp()
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** File "/usr/lib/python3.7/ftplib.py", line 244, in getresp
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** raise error_temp(resp)
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** ftplib.error_temp: 451-Error during read from data connection
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** 451-Transfer aborted
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: **** 451 0.082 seconds (measured here), 194.36 Kbytes per second
Sep 5 15:26:07 weatherpi weewx[30940] ERROR weewx.reportengine: ftpgenerator: Upload failed