Here is one of my jstyles. All of my styles give me the above error, but I tested other styles, without success (I used already them in previous JabRef versions).
# This file is licensed under the terms of the GPL version 2 (or later).
NAME
Literatura - Eng
JOURNALS
Literatura
PROPERTIES
Title="References"
IsSortByPosition="false"
IsNumberEntries="false"
ReferenceParagraphFormat="List 1"
ReferenceHeaderParagraphFormat="Heading 1"
# Configurare Heading1: Italics not capitalized, left justified
CITATION
AuthorField="author/editor"
YearField="year"
MaxAuthors="3"
MaxAuthorsFirst="3"
AuthorSeparator=", "
AuthorLastSeparator=" & "
AuthorLastSeparatorInText=" & "
EtAlString=" and oth."
ItalicEtAl="false"
YearSeparator=" "
InTextYearSeparator=" "
BracketBefore=
BracketAfter=
BracketBeforeInList="["
BracketAfterInList="]"
CitationSeparator="; "
UniquefierSeparator=", "
GroupedNumbersSeparator="-"
MinimumGroupingCount="3"
FormatCitations="false"
CitationCharacterFormat="Default"
MultiCiteChronological="false"
PageInfoSeparator=", "
LAYOUT
article=\format[AuthorLastFirst]{\author}. \year. \format[HTMLChars,FormatPagesForXML]{\title}. <i>\journal</i>\begin{volume} \volume\end{volume}\begin{number}(\number)\end{number}, \format[FormatPagesForHTML]{\pages}
book=\begin{author}\format[Authors(FullName,LastFirstFirstFirst,Amp)]{\author}. \end{author}\begin{editor}\format[AuthorLastFirstFirstFirst,AuthorAndsReplacer]{\editor}, \format[IfPlural(eds,ed)]{\editor}. \end{editor}\year. <i>\format[HTMLChars,FormatPagesForHTML]{\title}.</i>\begin{subtitle} \format[HTMLChars,FormatPagesForHTML]{\subtitle}.\end{subtitle}\begin{series&&number} \series \number.\end{series&&number}\begin{volume} \format[HTMLChars,FormatPagesForHTML]{\volume}.\end{volume}\begin{volumes} \volumes vol.\end{volumes}\begin{location} \format[HTMLChars,FormatPagesForHTML]{\location}: \end{location}\begin{place}\format[HTMLChars]{\place}: \end{place}\begin{edition}<sup>\edition</sup>\end{edition}\publisher
inbook=\format[Authors(FullName,LastFirstFirstFirst,Amp)]{\author}. \format[FormatPagesForHTML]{\year}: \format[HTMLChars,FormatPagesForHTML]{\title}. <i>\format[HTMLChars,FormatPagesForHTML]{\booktitle}. \begin{booksubtitle} \format[HTMLChars,FormatPagesForHTML]{\booksubtitle}. \end{booksubtitle}</i>\format[HTMLChars,Authors(FullName,FirstFirst,Amp)]{\editor}, \format[IfPlural(eds,ed)]{\editor}.\begin{location} \format[HTMLChars,FormatPagesForHTML]{\location}: \end{location}\begin{place}\format[HTMLChars]{\place}: \end{place}\begin{edition}<sup>\edition</sup>\end{edition}\publisher\begin{pages}, \format[FormatPagesForHTML]{\pages}\end{pages}
inproceedings=\format[Authors(FullName,LastFirstFirstFirst,Amp)]{\author}. \format[FormatPagesForHTML]{\year}: \format[HTMLChars,FormatPagesForHTML]{\title}. <i>\format[HTMLChars,FormatPagesForHTML]{\eventtitle}.</i> \begin{eventtitleaddon}\format[HTMLChars,FormatPagesForHTML]{\eventtitleaddon}. \end{eventtitleaddon}\format[HTMLChars,Authors(FullName,FirstFirst,Amp)]{\editor}, \format[IfPlural(eds,ed)]{\editor}.\begin{location} \format[HTMLChars,FormatPagesForHTML]{\location}: \end{location}\begin{place}\format[HTMLChars]{\place}: \end{place}\begin{edition}<sup>\edition</sup>\end{edition}\publisher\begin{pages}, \format[FormatPagesForHTML]{\pages}\end{pages}
incollection=\format[Authors(FullName,LastFirstFirstFirst,Amp)]{\author}. \format[FormatPagesForHTML]{\year}: \format[HTMLChars,FormatPagesForHTML]{\title}. <i>\format[HTMLChars,FormatPagesForHTML]{\booktitle}. \begin{booksubtitle} \format[HTMLChars,FormatPagesForHTML]{\booksubtitle}. \end{booksubtitle}</i>\format[HTMLChars,Authors(FullName,FirstFirst,Amp)]{\editor}, \format[IfPlural(eds,ed)]{\editor}.\begin{location} \format[HTMLChars,FormatPagesForHTML]{\location}: \end{location}\begin{place}\format[HTMLChars]{\place}: \end{place}\begin{edition}<sup>\edition</sup>\end{edition}\publisher\begin{pages}, \format[FormatPagesForHTML]{\pages}\end{pages}
#####
phdthesis=\format[AuthorLastFirst,AuthorAbbreviator,AuthorAndsReplacer]{\author}
(<b>\year\uniq</b>). <i>\title</i>, \school.
default=\format[AuthorLastFirst]{\author}. \year. \format[HTMLChars,FormatPagesForXML]{\title}. <i>\journal</i>\begin{volume} \volume\end{volume}\begin{number}(\number)\end{number}, \format[FormatPagesForHTML]{\pages}
JabRef 5.0.0-dev--2019-10-15----2a5507fd3
Linux 5.2.0-0.bpo.3-amd64 amd64 (Debian 10 Buster)
Java 12.0.2
Steps to reproduce the behavior:
Log File
Here is one of my jstyles. All of my styles give me the above error, but I tested other styles, without success (I used already them in previous JabRef versions).