The template provides two alternatives in those cases, a singular and a plural, while some upstream Czech translator has provided three alternatives, for instance:
So LP refuses to import, and is apparently confused also when you try to fix it via the LP GUI.
I think you should resolve this with the upstream Czech translator(s).
In the meantime you may want to make an experiment: Download the Czech PO file, add translations for msgstr[0] and msgstr[1], remove the lines with msgstr[2], and upload the modified PO file manually.
The template provides two alternatives in those cases, a singular and a plural, while some upstream Czech translator has provided three alternatives, for instance:
msgstr[0] "před minutou"
msgstr[1] "před %d minutami"
msgstr[2] "před %d minutami"
So LP refuses to import, and is apparently confused also when you try to fix it via the LP GUI.
I think you should resolve this with the upstream Czech translator(s).
In the meantime you may want to make an experiment: Download the Czech PO file, add translations for msgstr[0] and msgstr[1], remove the lines with msgstr[2], and upload the modified PO file manually.