@daniel-molina The input file is an invalid GPX file see:

https://codeberg.org/jannis/FitoTrack/issues/321

Maybe Viking could be more robust against such input

I have not much experience on GPX, but xmlvalidation.com firstly reports for my previously attached FitoTrack GPX to have a warning. However, later it refers to it as an error. In any case, it is just what is referred in the FitoTrack issue that you mentioned.

A warning has been found!
[...]
Errors in the XML document:
	2:	304	schema_reference.4: Failed to read schema document 'http://www.topografix.com/GPX/1/1/gpx.xsd', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not <xsd:schema>.

I can also say that OSM accepts FitoTrack GPX files as input (it is not possible to do it from viking but directly from OSM web interface). Personally, it would be nice for me if viking could manage those files :D, but I understand that is FitoTrack issue after all.

I tried an alternative to FitoTrack, the Trip Recording plugin of OsmAnd~. After exporting the GPX with viking new errors appear. The original GPX is attached in a OsmAnd issue I reported, currently being discussed there if the trace is invalid or not. Output from viking for that input is attached here.

BusGranadaZubia_viking.txt

I think ATM if one uses the Track-->Export Track as GPX..., then Viking writes it's own GPX header - which may not be consistent with the trackpoint extensions being used (which in the the above case has osmand:speed value extensions).
And so may fail 'external schema' validation.

NB When one uses the Layer-->Export Layer-->Export as GPX... then the input GPX headers are retained properly and the so the exported file should be consistent.